The Degradation of CoA: Subcellular Localization and Kinetic Properties of CoA‐and Dephospho‐CoA Pyrophosphatase

An enzyme degrading dephospho‐CoA with 5′‐AMP as the reaction product is present in the plasma membranes of rat liver. CoA is degraded at less than 10% of the rate of dephospho‐CoA. 2. The presence of plasma membranes in nuclear and microsomal fractions, which also contain significant amounts of the plasma‐membrane marker enzymes, probably explains the presence of dephospho‐CoA pyrophosphatase in these fractions. The enzyme is different from inorganic pyrophosphatase present in nuclei and microsomes with respect to pH optimum and sensitivity to inhibitors. 3. Dephospho‐CoA pyrophosphatase is inactivated by dialysis against EDTA, and is partially reactivated by Mn2+ and a number of other divalent cations. 4. The pyrophosphatase degrading dephospho‐CoA and CoA is probably identical with nucleotide pyrophosphatase from plasma membranes. The degradation of dephospho‐CoA is inhibited competitively by nucleotides with a pyrophosphate bond; most strongly by NADH, which has the highest affinity for nucleotide pyrophosphatase. 5. Competitive inhibitors of dephospho‐CoA degradation are less efficient when a ribose 3′‐phosphate is present in the nucleotide. Km for dephospho‐CoA is 0.02 mM, for CoA about 0.3 mM. These results indicate that the 3′‐phosphate group is a sterical hindrance for the access of the nucleotides to the active site. V for dephospho‐CoA in plasma membranes was about 200 nmol×mg protein−1×min−1, but for CoA one order of magnitude less. 6. Also nucleotide pyrophosphatase from Crotalus adamanteus degraded CoA at a much lower rate than dephospho‐CoA (below 1%).