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Physiology of Absorption, Metabolism, and Excretion

Absorption

CoA in the diet is hydrolyzed in the intestinal lumen to dephospho CoA, phosphopantetheine, and pantetheine, with the pantetheine subsequently hydrolyzed to pantothenic acid (Shibata et al., 1983). Pantothenic acid was the only one of these pantothenate-containing compounds absorbed by rats in studies on absorption of the various forms. Absorption is by active transport at low concentrations of the vitamin and by passive transport at higher concentrations in animal models (Fenstermacher and Rose, 1986). Because the active transport system is saturable, absorption will be less efficient at higher concentrations of intake, but the intake levels at which absorptive efficiency decreases in humans are not known. Intestinal microflora have been observed to synthesize pantothenic acid in mice (Stein and Diamond, 1989), but the contribution of bacterial synthesis to body pantothenic acid levels or fecal losses in humans has not been quantified. If microbial synthesis is substantial, balance studies in humans may have underestimated pantothenic acid absorption and requirements.

Metabolism

The synthesis of CoA from pantothenate is regulated primarily by pantothenate kinase, an enzyme that is inhibited by the pathway end products, CoA and acyl CoA. Thus CoA production does not reflect the amount of available pantothenate (Tahiliani and Beinlich, 1991). CoA, in forms such as acetyl CoA and succinyl CoA, plays an important role in the tricarboxylic acid cycle and in the synthesis of fatty acids and membrane phospholipids, amino acids, steroid hormones, vitamins A and D, porphyrin and corrin rings, and neurotransmitters. It is also required for the acetylation and acylation of proteins and the synthesis of α-tubulin (Plesofsky-Vig, 1996).

Excretion

CoA is hydrolyzed to pantothenate in a multiple-step reaction. The pantothenic acid is excreted intact in urine, where it can be measured by using a Lactobacillus plantarum assay or a radioimmunoassay. The amount excreted varies proportionally with dietary intake over a discrete yet wide range of intake values.



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