C" always refers to the -enantiomer of ascorbic acid and its oxidized forms, such as dehydroascorbate (DHA). Therefore, unless written otherwise, "ascorbate" and "ascorbic acid" refer in the nutritional literature to -ascorbate and -ascorbic acid respectively. Ascorbic acid is a weak sugar acid structurally related to glucose. In biological systems, ascorbic acid can be found only at low pH, but in solutions above pH 5 is predominantly found in the ionized form, ascorbate. All of these molecules have vitamin C activity and thus are used synonymously with vitamin C, unless otherwise specified. Numerous analytical methods have been developed for ascorbic acid detection. For example, vitamin C content of a food sample such as fruit juice can be calculated by measuring the volume of the sample required to decolorize a solution of dichlorophenolindophenol (DCPIP) and then calibrating the results by comparison with a known concentration of vitamin C. Testing Simple tests are available to measure the levels of vitamin C in urine and serum. These better reflect recent dietary intake rather than total body content. It has been observed that while serum concentrations follow a circadian rhythm or reflect short-term dietary influence, content within cells or tissues is more stable and can give a better view of the availability of ascorbate within the entire organism. However, very few hospital laboratories are adequately equipped and trained to carry out such detailed analyses. Synthesis Most animals and plants are able to synthesize vitamin C through a sequence of enzyme-driven steps, which convert monosaccharides to vitamin C. Yeasts do not make -ascorbic acid but rather its stereoisomer, erythorbic acid. In plants, synthesis is accomplished through the conversion of mannose or galactose to ascorbic acid. In animals, the starting material is glucose. In some species that synthesize ascorbate in the liver (including mammals and perching birds), the glucose is extracted from glycogen; ascorbate synthesis is a glycogenolysis-dependent process. In humans and in animals that cannot synthesize vitamin C, the enzyme -gulonolactone oxidase (GULO), which catalyses the last step in the biosynthesis, is highly mutated and non-functional. Animal synthesis There is some information on serum vitamin C concentrations maintained in animal species that are able to synthesize vitamin C. One study of several breeds of dogs reported an average of 35.9 μmol/L. A report on goats, sheep and cattle reported ranges of 100–110, 265-270 and 160-350 μmol/L, respectively. The biosynthesis of ascorbic acid in vertebrates starts