IngredientsL-Gulonolactone oxidase — formally designated L-gulono-γ-lactone oxidase, or L-gulono-1,4-lactone oxidase — is an enzyme of central importance to vertebrate biochemistry. It bears the Enzyme Commission number EC 1.1.3.8 and is expressed in most mammals, but is non-functional in Haplorrhini (a suborder of primates, including humans), in some bats, and in guinea pigs. The gene encoding it is abbreviated GULO; in humans the corresponding locus exists only as a non-functional remnant, designated GULOP (the pseudogene).
The GULO enzyme belongs to the family of aldonolactone oxidoreductases (AlORs), which are a part of the vanillyl alcohol oxidase (VAO) family of flavoenzymes. It contains two conserved domains: an N-terminal FAD-binding region and a C-terminal HWXK motif capable of binding the flavin cofactor.
The enzyme catalyzes the reaction of L-gulono-1,4-lactone with oxygen to form L-xylo-hex-3-gulonolactone (2-keto-gulono-γ-lactone) and hydrogen peroxide, using FAD as a cofactor. The L-xylo-hex-3-gulonolactone then converts to ascorbic acid spontaneously, without further enzymatic action.
In the context of supplementation and natural products science, "gulonolactone oxidase" is occasionally referenced as both the enzyme itself and — more practically — its enzymatic substrate L-gulono-1,4-lactone (gulonolactone), which is a naturally occurring intermediate in vitamin C biosynthesis. Hydrolysis of glucuronic acid and NADPH-dependent reduction lead to L-gulonic acid, which spontaneously cyclizes to L-gulonolactone; in many animals and higher plants, this can be converted to 2-ketogulonolactone — a precursor of ascorbic acid (vitamin C) — by gulonolactone oxidase.
The enzyme is present only in the liver of the mouse, rat, pig, cow, and dog among common laboratory and domestic animals. Vitamin C is synthesized in the liver in most mammals and in the kidney in birds and reptiles.
Although numerous species are capable of synthesizing vitamin C, this enzymatic activity is lost in several mammals, such as humans, other primates, guinea pigs, some bat species, and insects. Many species, such as teleost fishes, anthropoid primates, guinea pigs, as well as some bat and Passeriformes bird species, have lost the capacity to synthesize it.
In all cases so far studied, the inability to synthesize vitamin C is due to mutations in the L-gulono-γ-lactone oxidase (GLO) gene, which codes for the enzyme responsible for catalyzing the last step of vitamin C biosynthesis. The bias for mutations in this particular gene is likely due to the fact that losing it only affects vitamin C production.
Gene structure and phylogenetic analyses showed that vertebrate GULO genes are 64–95% identical at the amino acid level and consist of 11 conserved exons. In animals where GULO is functional, the enzyme is localized in specific tissues: the enzyme is associated with the endoplasmic reticulum membrane.
This enzyme family is a subfamily under the broader sugar-1,4-lactone oxidases, which also includes the bacterial L-gulono-1,4-lactone dehydrogenase and the plant galactonolactone dehydrogenase. All these aldonolactone oxidoreductases play a role in some form of vitamin C synthesis, and some accept substrates of other members. Bacteria also express closely related enzymes: a BLAST search using the rat L-gulono-1,4-lactone oxidase sequence revealed the presence of closely related orthologs in a limited number of bacterial species, including several pathogens of human lungs, such as Mycobacterium tuberculosis, Pseudomonas aeruginosa, Burkholderia cepacia and Bacillus anthracis.
About 61 million years ago, some mammals and primates, including human ancestors, lost the ability for endogenous vitamin C synthesis due to the inactivation of the l-gulono-lactone oxidase (GLO) gene, with the consequence that the last step of the ascorbate synthesis from glucose was blocked. From then on, these species, including some primates, guinea pigs and Indian fruit bats, became dependent on daily dietary intake of ascorbic acid.
Humans lost this capability to synthesize vitamin C endogenously, about 40 million years ago, due to the development of mutations in the gulonolactone oxidase gene, stopping the vitamin C biosynthetic pathway. The human gene is a remnant that lacks five of twelve exons found in functional rodent genes. The non-functional gulonolactone oxidase pseudogene (GULOP) was mapped to human chromosome 8p21, which corresponds to an evolutionarily conserved segment on either porcine chromosome 4 (SSC4) or 14 (SSC14).
GULO pseudogenes have multiple indel mutations and premature stop codons in higher primates, guinea pigs, and some bats. The loss of this enzyme activity is responsible for the inability of guinea pigs to enzymatically synthesize vitamin C. Both these events happened independently of the loss in the haplorrhine suborder of primates, which includes humans. The remnant of this non-functional gene with many mutations is still present in the genomes of guinea pigs and humans.
Whereas the GLO gene mutations in fish, anthropoid primates and guinea pigs are irreversible, some of the GLO pseudogenes found in bat species have been shown to be reactivated during evolution. The same phenomenon is thought to have occurred in some Passeriformes bird species. Interestingly, these GLO gene losses and reactivations are unrelated to the diet of the species involved, suggesting that losing the ability to make vitamin C is a neutral trait.
There is ongoing discussion about the benefit of the inactivation of GLO and the selective pressures on this phenotype. One hypothesis — the "ascorbate-rich diet hypothesis" — tries to explain the inactivation of l-gulono-lactone oxidase through the presence of adequate vitamin C within the diet.
GULO encodes the final protein for de novo vitamin C synthesis, and its loss is thought to occur as a neutral mutation in some organisms which can acquire sufficient dietary ascorbic acid.
An evolutionary improvement in the control of redox homeostasis was also considered, as potentially toxic H₂O₂ is generated as a byproduct in the vitamin C biosynthesis pathway. In other words, eliminating a source of endogenous hydrogen peroxide may have conferred a selective advantage in some lineages.
Linus Pauling observed that after the loss of endogenous ascorbate production, apo(a) and Lp(a) were greatly favored by evolution, acting as ascorbate surrogates, since the frequency of occurrence of elevated Lp(a) plasma levels in species that had lost the ability to synthesize ascorbate is great.
L-Gulonolactone oxidase deficiency has been called "hypoascorbemia" and is described by OMIM (Online Mendelian Inheritance in Man) as "a public inborn error of metabolism," as it affects all humans. The inactivation of the GULO gene and the subsequent reliance on dietary vitamin C may have broader implications for aging and age-related diseases, as one of the most important actions of vitamin C is as an antioxidant.
There is no historical tradition of directly supplementing GULO enzyme itself, as the concept of the enzyme was not known until the modern biochemical era. However, the clinical consequence of GULO dysfunction — scurvy — has been recognized across millennia. The lack of vitamin C (known traditionally as "scurvy") results in a host of systemic defects within susceptible individuals, but the most obvious are the result of blood vessel fragility. The vitamin C deficiency leads to an impaired synthesis of collagen secondary to defective hydroxylase reactions in the formation of hydroxylysine and hydroxyproline amino acids.
Vitamin C was first isolated in 1928 by Albert Szent-Györgyi, using lemons, pepper, and adrenal glands as extraction substrates. The biochemical identification of L-gulonolactone oxidase as the responsible enzyme came considerably later, through molecular cloning and enzymatic characterization studies in the mid-to-late twentieth century. The characterization of the rat liver GULO enzyme via complementary DNA isolation and sequencing was reported by Koshizaka, Nishikimi, Ozawa, and Yagi, establishing the molecular basis of the biosynthetic pathway.
The last step in the pathway of vitamin C synthesis is the oxidation of l-gulonolactone to l-ascorbic acid by l-gulonolactone oxidase, an enzyme associated with the endoplasmic reticulum membrane and deficient in man, guinea pig and other species. It is synthesized, in vertebrates having this capacity, from d-glucuronate.
Glucuronate is converted to l-gulonate by aldehyde reductase, an enzyme of the aldo-keto reductase superfamily. L-Gulonate is converted to l-gulonolactone by a lactonase identified as SMP30 or regucalcin, whose absence in mice leads to vitamin C deficiency. The last step in the pathway is the oxidation of l-gulonolactone to l-ascorbic acid by l-gulonolactone oxidase.
When hydrogen peroxide is produced as a consequence of the oxidation of an aldonolactone using molecular oxygen as an electron acceptor, this family of enzymes is known as oxidases. Specifically, the enzyme catalyzes the reaction of L-gulono-1,4-lactone with oxygen to form L-xylo-hex-3-gulonolactone (2-keto-gulono-γ-lactone) and hydrogen peroxide. It uses FAD as a cofactor. The L-xylo-hex-3-gulonolactone then converts to ascorbic acid spontaneously, without enzymatic action.
The GULO enzyme belongs to the family of aldonolactone oxidoreductases (AlORs) and contains two conserved domains: an N-terminal FAD-binding region and a C-terminal HWXK motif capable of binding the flavin cofactor. A recombinant C-terminal rat GULO expressed in Escherichia coli demonstrated enzymatic activity, suggesting that the binding of the flavin cofactor to the HWXK motif at the C-terminus is sufficient for the formation of the enzyme's active site.
Recombinant GULO variants showed their highest enzymatic activity at pH levels of 7 and 6.5 and temperatures of 40°C and 30°C for the full and C-terminal forms, respectively. In terms of stability, the enzyme showed optimal stability at pH levels between 6.5 and 8.5, as well as temperatures from 20 to 40°C.
The sole known product of GULO's catalytic activity is L-ascorbic acid (vitamin C). The physiological functions attributed to GULO deficiency are therefore mediated entirely through ascorbate insufficiency. Ascorbic acid is an important antioxidant and reducing agent. It is required for the hydroxylation of proline and lysine, a process that is essential in the formation of collagen.
Other functions of ascorbic acid include the synthesis of carnitine, neurotransmitters and the catabolism of tyrosine, among others.
Ascorbic acid is also required in vivo as a cofactor for enzymes involved in carnitine and catecholamine norepinephrine biosynthesis, peptide amidation, and tyrosine catabolism. Moreover, as an enzymatic cofactor, vitamin C is involved in processes of gene transcription and epigenetic regulation.
Ascorbic acid is known for its antioxidant properties, which help protect against reactive oxygen species generated from metabolic activities; however, at high doses, it may exhibit pro-oxidative effects.
It is important to note that L-gulonolactone oxidase itself is not available as a conventional dietary supplement for humans, since the human body lacks the functional enzyme and its substrate (L-gulono-1,4-lactone) is not a standard commercially available supplement. The scientific evidence reviewed below therefore addresses: (1) the consequences of GULO gene deficiency as studied in animal models; (2) experimental attempts to restore GULO function in human cells and GULO-knockout animals; and (3) the clinical implications of GULO absence for human health.
Animal models have provided the strongest direct evidence linking GULO deficiency to skeletal pathology. Using a mouse mutant that fractures spontaneously and dies at a very young age, researchers identified that a deletion of the GULO gene, which is involved in the synthesis of vitamin C, is the cause of impaired osteoblast differentiation, reduced bone formation, and development of spontaneous fractures.
Femur and tibial bone mineral density were reduced by 27% and 36%, respectively, in these sfx mice at 5 weeks of age. Histomorphometric analyses of bones from sfx mice revealed that bone formation rate was reduced by more than 90% and was caused by impairment of differentiated functions of osteoblasts. The conclusion was that the sfx is a mutation of the GULO gene, which leads to ascorbic acid deficiency, impaired osteoblast cell function, and fractures in affected mice.
Evidence strength: Preclinical (animal model) only. These findings are in line with long-established human clinical knowledge about scurvy-related bone disease, but direct GULO-intervention studies in humans do not exist in this context.
Even though a hypovitaminosis C condition may not lead to scurvy, it places an individual at higher risk for metabolic abnormalities, cardiovascular diseases, and cancer. Several studies on vitamin C deficient Gulo⁻/⁻ mice have indicated that low levels of vitamin C increase aortic wall damage and sensorimotor deficits, induce impaired neutrophil apoptosis and clearance, deteriorate bone microarchitecture, and decrease the life span of these mice.
Low blood levels of ascorbate have been inversely related to several chronic diseases and their associated risk factors, including C-reactive protein, blood pressure, diabetes, metabolic syndrome, cardiovascular diseases, and all-cause mortality.
The exonal knockout that inactivates the enzyme requires ascorbate supplementation to maintain viability in these mice. Additional studies on this knockout model indicated elevated oxidative stress and sensorimotor deficits as well as behavioral and monoamine changes following severe ascorbate deficiency.
Evidence strength: Largely preclinical (Gulo⁻/⁻ mice), supported by observational human epidemiology on ascorbate status and disease risk. No human trials directly administering GULO enzyme or gulonolactone as a supplement exist.
Anthropoid primates and guinea pigs have lost the ability to synthesize vitamin C due to mutations in the l-gulono-γ-lactone oxidase (GULO) gene, which codes for the enzyme responsible for catalyzing the last step of vitamin C biosynthesis. The inability to synthesize vitamin C may partly explain why humans and guinea pigs have an increased vulnerability to sepsis and to dying from sepsis.
The inability to generate vitamin C makes humans very susceptible to dysfunction in a variety of biochemical pathways that are vital for surviving a critical illness such as sepsis.
L-Gulono-1,4-lactone oxidase is missing in scurvy-prone, vitamin C-deficient animals, such as humans and guinea pigs, which are also highly susceptible to tuberculosis.
Evidence strength: Mechanistic and observational. Animal model data are supported by clinical observations linking low vitamin C to impaired immunity. No clinical trials have directly supplemented GULO enzyme.
Research in cell culture has explored restoring GULO function to cancer cells as a strategy for modulating ascorbate-dependent oncogenic pathways. Humans are unable to synthesise ascorbate due to the lack of a functional gulonolactone oxidase (GULO), the enzyme that catalyses the final step in the biosynthesis pathway. Ascorbate is a vital micronutrient required for many biological functions, including as a cofactor for metalloenzymes that regulate the transcription factor hypoxia-inducible factor-1 (HIF-1), which governs cell survival under hypoxia.
This study aimed to restore ascorbate synthesis to human hepatocellular carcinoma HepG2 cells and determine the effect of internally produced ascorbate on HIF-1 activation. HepG2 cells were gene-modified with a plasmid encoding the mouse Gulo cDNA. A PCR-positive clone synthesised ascorbate when the Gulo substrate, l-gulono-1,4-lactone, was supplied. Intracellular ascorbate concentrations reached 5% of saturation levels (6 nmol/10⁶ cells). Addition of ascorbate or gulonolactone reduced HIF-1 accumulation in the Gulo clone, but also in parental HepG2 cells.
This study confirmed that gene transfer of the mouse gulonolactone oxidase encoding gene was insufficient in itself to restore ascorbate synthesis to human cells. Initial evidence is provided that the ascorbate precursor, gulonolactone, may dampen the HIF-1 response in HepG2 cells.
The data indicate that the ascorbate biosynthesis pathway in human cells has been significantly modified and may contain numerous non-functional members besides gulonolactone oxidase. Future biochemical studies will be needed to determine which other enzymes in the pathway are non-functional.
Evidence strength: In vitro (cell culture) only. Results are preliminary and exploratory. No human or animal in vivo trials have followed from this line of research as a direct therapeutic intervention.
A 2025 preprint study used Gulo⁻/⁻ mice to examine the role of ascorbate in parasitic infection. It is assumed that loss of ascorbate synthesis incurs no fitness costs because it can be fully replaced by dietary intake, and GULO loss is a paradigmatic evolutionarily neutral gene loss. However, unlike ascorbate-synthesizing organisms such as mice which maintain uniformly high systemic ascorbate levels, GULO loss allows for large variations in plasma ascorbate levels.
Evidence strength: Very preliminary; preprint animal-model data only.
A principal area of translational research has been exploring whether GULO function could be re-introduced into GULO-deficient organisms as a therapeutic strategy. Inability to synthesize vitamin C because of a deficiency in gulonolactone oxidase (GULO) expression is a genetic deficiency shared by a small number of animals including humans. Although the most overt symptom of vitamin C deficiency, scurvy, can be readily corrected by modest consumption of vitamin C, there is increasing interest in the effect of high-level administration in treating human disease.
Gulo⁻/⁻ mice treated with a helper-dependent adenovirus-based GULO-expressing vector expressed GULO in the liver and produced ascorbic acid. Serum ascorbic acid concentrations in these mice were elevated to levels comparable to those of wild-type mice (62 ± 15 µM) after 4 days of infection and were maintained at significantly higher levels compared with untreated Gulo⁻/⁻ mice for at least 23 days. A similar elevation was observed in urine and tissue ascorbic acid concentrations in vector-treated animals.
Vitamin C deficiency is an excellent candidate for therapeutic intervention by ectopic gene expression because a single gene mutation is responsible for the condition and, it is known, a priori, that supplementation effectively rescues the deficiency (at least with respect to the development of scurvy). There are several strategies to ectopically express exogenous genes or silence endogenous genes, including viruses and nucleic acids.
Man, with other primates, lost the ability to synthesize vitamin C through an inactivating mutation of the gene encoding gulonolactone oxidase (GULO) millions of years ago. Though the consequences of this prehistoric loss must have been favorable (and thus selected for) at the population level, the inability to produce vitamin C may have serious health implications for modern humans, especially for those conditions in which antioxidants like vitamin C have been implicated as potential therapeutic agents.
Evidence strength: Preclinical (animal and cell culture). No human gene therapy trials have been conducted to restore GULO function as of the available literature.
Because GULO enzyme is not commercially available as a direct oral supplement for humans, dosage information in the literature pertains primarily to: (1) ascorbate supplementation used in GULO-knockout animal experiments to model vitamin C repletion; (2) the substrate gulonolactone used in cell culture; and (3) gene vector doses in gene therapy experiments. No human clinical trials have administered GULO enzyme or its immediate substrate as an oral supplement.
There are no established safety data for oral administration of isolated L-gulonolactone oxidase enzyme or its substrate L-gulono-1,4-lactone to humans, since no such human supplementation studies have been conducted. The cell-culture and mouse gene therapy literature does not translate to safety data for human dietary supplement use.
Low rescue efficiency of Gulo-expressing adenoviral constructs and reduced viral growth in HEK293 cells were observed, suggesting that overexpression of Gulo may be inhibitory to cell growth. This is a preliminary finding in cell culture that may have implications for potential gene therapy applications but has no direct relevance to dietary supplement use.
An evolutionary improvement in the control of redox homeostasis was considered as a possible rationale for GULO gene loss, as potentially toxic H₂O₂ is generated as a byproduct in the vitamin C biosynthesis pathway. This observation underscores that GULO's catalytic activity is not without potential cellular consequences, and any future strategy to restore GULO function in humans would need to account for peroxide generation.
The development of a genetically modified hepatic cell line resulted in a human cell line that could produce its own ascorbate, but only when gulonolactone was supplied externally. This data indicates that the ascorbate biosynthesis pathway in human cells has been significantly modified and may contain numerous non-functional members besides gulonolactone oxidase. Future biochemical studies will determine which other enzymes in the pathway are non-functional.
This finding is significant because it means that even if GULO enzyme activity were to be restored in human cells, additional enzymatic deficiencies earlier in the pathway — related to the conversion of glucose through to gulonolactone — would also need to be corrected before endogenous ascorbate synthesis could be fully re-established.
Ascorbic acid, the product of GULO activity, is known for its antioxidant properties which help protect against reactive oxygen species generated from metabolic activities; however, at high doses, it may exhibit pro-oxidative effects. This dual property of the product is relevant context for any theoretical restoration of GULO activity that might produce supraphysiological ascorbate concentrations.
More than 30 years ago, Ames and colleagues hypothesized that higher serum uric acid levels might have been beneficial during hominoid evolution because of the antioxidant properties of uric acid. Loss of l-gulonolactone oxidase, the enzyme responsible for ascorbic acid synthesis, preceded the loss of uricase during primate evolution and may have raised the selection pressure for augmentation of an already existing alternative antioxidant system.
Though the consequences of the prehistoric loss of GULO must have been favorable at the population level, the inability to produce vitamin C may have serious health implications for modern humans, especially for those conditions in which antioxidants like vitamin C have been implicated as potential therapeutic agents.
Vitamin C is now known to be involved in several novel physiological phenomena including stem cell differentiation and respiratory development, which likely require pharmacological levels of vitamin C.
Research on GULO is currently concentrated in three main domains: (1) evolutionary genomics and comparative biology, tracing the pattern of GULO gene losses and rare reactivations across vertebrate lineages; (2) biotechnology, where the GULO enzyme and its substrate are being explored as components in biosynthetic pathways for industrial vitamin C production from glucose; and (3) translational medicine, examining whether gene therapy approaches can functionally restore ascorbate biosynthesis in GULO-deficient organisms.
L-gulonolactone oxidase (GULO) is required for the final step of L-ascorbic acid biosynthesis. The commercial production of vitamin C has typically relied on the Reichstein process and two-step methods. However, both methods have a similar drawback: vitamin C cannot be produced directly from D-glucose. An innovative technique for producing vitamin C from D-glucose in E. coli was introduced using a single-step fermentation process, utilizing the expression of ten genes from Arabidopsis thaliana involved in the vitamin C biosynthesis pathway.
Elevating vitamin C content by overexpressing inositol oxygenase and gulono-1,4-lactone oxidase in A. thaliana leads to enhanced biomass and tolerance to abiotic stresses.
As of the current literature, there are no marketed dietary supplements that deliver L-gulonolactone oxidase enzyme to humans in a form that could restore endogenous vitamin C biosynthesis, and no peer-reviewed human clinical trials have tested such an approach. The principal clinical relevance of GULO research remains the understanding of why humans must obtain vitamin C entirely from dietary sources, and what the health consequences of inadequate intake are.
Health conditions that Gulonolactone oxidase may help support.
Body systems that Gulonolactone oxidase may help support.
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