Properties of the medium chain/long chain carnitine acyltransferase purified from rat liver microsomes

Carnitine octanoyltransferase (COT) purified from rat liver microsomes has K0.5 values between 1.0 and 4.0 microM for saturated 6-carbon to 16-carbon length acyl-CoAs, with little differences in Vmax values. The reaction rate is linear with time in the forward direction (acyl-CoA-->acylcarnitine), but it increases with time when assayed in the reverse direction (acylcarnitine-->acyl-CoA). The K0.5 for decanoylcarnitine and CoASH are 0.3 mM for CoASH and between 1.0 and 4.0 mM for decanoylcarnitine. The kinetic data indicate that the enzyme functions in the direction of acyl-carnitine formation. It is moderately inhibited by aminocarnitine, and D-carnitine and etomoxiryl-CoA are weak inhibitors; malonyl-CoA does not inhibit the enzyme. The enzyme has little, if any, capacity to use valproylcarnitine, 3-methylglutarylcarnitine, or pivaloylcarnitine as a substrate. Polyclonal antibodies prepared against COT give a positive Western blot against the purified enzyme and against a protein in microsomes having the molecular mass of COT (53 kDA). Antimitochondrial CPT and antiperoxisomal CAT did not show appreciable cross-reactivity with purified microsomal COT. The inhibitor data, the kinetic data, the molecular masses, and the Western blotting profiles all show that the enzyme purified from rat liver microsomes is a different carnitine acyltransferase than those previously purified from other organelles.