Catabolism of CoA

CoA undergoes dephosphorylation, catalyzed by lysosomal acid phosphatase, to dephospho-CoA, followed by pyrophosphatase action to release 4'-phosphopantetheine and 5 -AMP - the reverse of the final stages of CoA synthesis shown in Figure 12.2. CoA is also a substrate for direct pyrophosphatase action, at about 10% of the rate of action on dephospho-CoA. The pyrophosphatase seems to be a general nucleotide pyrophosphatase of plasma membrane rather than an enzyme specific for the degradation of CoA.

Phosphopantetheine, arising from either the catabolism of CoA or the inac-tivation of holo-acyl carrier protein (ACP), can be reutilized for CoA synthesis. Phosphopantetheine is a potent inhibitor of pantothenic acid kinase, the first step of de novo CoA synthesis.

Alternatively, phosphopantetheine is dephosphorylated, again by a relatively unspecific phosphatase. The resultant pantetheine is cleaved by pante-theinase, a specific amidase, to pantothenic acid and cysteamine. The resultant cysteamine may be an important precursor of taurine (Section 14.5.1). Pan-tetheinase is found in both the liver and kidneys. The kidney isoenzyme acts on both pantetheine and (at a lower rate) on phosphopantetheine, whereas the liver enzyme acts only on pantetheine (Dupreetal., 1973; Wittweretal., 1983).

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