Pig muscle carnitine palmitoyltransferase I (CPTI beta), with low Km for carnitine and low sensitivity to malonyl-CoA inhibition, has kinetic characteristics similar to those of the rat liver (CPTI alpha) enzyme.

The outer mitochondrial membrane enzyme carnitine palmitoyltransferase I (CPTI) catalyzes the initial and regulatory step in the beta-oxidation of long-chain fatty acids. There are two well-characterized isotypes of CPTI: CPTIalpha (also known as L-CPTI) and CPTIbeta (also known as M-CPTI) that in human and rat encode for enzymes with very different kinetic properties and sensitivity to malonyl-CoA inhibition. Kinetic hallmarks of the CPTIalpha are high affinity for carnitine and low sensitivity to malonyl-CoA inhibition, while the opposite characteristics, low affinity for carnitine and high sensitivity to malonyl-CoA, are intrinsic to the CPTIbeta isotype. We have isolated the pig CPTIbeta cDNA which encodes for a protein of 772 amino acids that shares extensive sequence identity with human (88%), rat (85%), and mouse (86%) CPTIbeta, while the degree of homology with the CPTIalpha from human (61%), rat (62%), and mouse (60%) is much lower. However, when expressed in the yeast Pichia pastoris, pig CPTIbeta shows kinetic characteristics similar to those of the CPTIalpha isotype. Thus, the pig CPTIbeta, unlike the corresponding human or rat enzyme, has a high affinity for carnitine (K(m) = 197 microM) and low sensitive to malonyl-CoA inhibition (IC(50) = 906 nM). Therefore, the recombinant pig CPTIbeta has unique kinetic characteristics, which makes it a useful model to study the structure-function relationship of the CPTI enzymes.