Dehydroascorbic acid DHA gives a pentacyclic dimer structure of indefinite stability. Recycling of ascorbate via active transport of DHA into cells, followed by reduction and reuse, mitigates the inability of humans to synthesize it from glucose. Vitamin C accumulates in mitochondria, where most of the free radicals are produced, by entering as DHA through the glucose transporters, GLUT10. Ascorbic acid protects the mitochondrial genome and membrane. Vitamin C does not pass from the bloodstream into the brain, although the brain is one of the organs that have the greatest concentration of vitamin C. Instead, DHA is transported through the blood–brain barrier via
L-gulonolactone oxidase deficiency living in a malarial infested environment". He based his observation on evidence which indicates that G6PD deficient individuals display enhanced sensitivity to ascorbic acid induced hemolysis. Studies of human diseases have benefited from the availability of small laboratory animal models. However, the tissues of animal models with a GULO gene generally have high levels of ascorbic acid and so are often only slightly influenced by exogenous vitamin C. Guinea pigs, for instance, are a popular human model but lost the ability to synthesize L-gulono-gamma-lactone oxidase 20 million years ago. This is a major handicap for studies involving the endogenous