Inhibition of rabbit muscle isozymes by vitamin C

The ability of vitamins C, E and K to inhibit enzymes directly has been inv estigated. It was found that vitamin E and some analogs and menadione (vita min K-3) inhibited several enzymes irreversibility at concentrations below one millimolar. Ascorbate inhibits rabbit muscle 6-phosphofructokinase (MPF K-1; EC 2.7.1.11), muscle type LDH (EC 1.1.1.27), and muscle AK (EC 2.7.4.3 ) at low concentrations that do not inhibit equivalent liver isozymes. Asco rbate K-i values for muscle-type LDH and heart-type LDH isozymes are 0.007 and 3 mM, respectively. The ascorbate K-i value for rabbit skeletal muscle PFK-1 is 0.16 mM; liver PFK-1 is not inhibited by ascorbate. Dehydroascorba te does not inhibit any enzyme at ascorbate concentrations normally found i n cells. All ascorbate inhibitions are completely reactivated or nearly so by L-ascorbate oxidase. CYS, GSH, or DTT. We propose a hypothesis that asco rbate facilitates glycogen storage in muscle by inhibiting glycolysis. The relationship between ascorbate metabolism and diabetes is discussed.