Inhibition of human lymphocyte ferrochelatase activity by hemin

Ferrochelatase activity in human lymphocytes was found to be 50% inhibited by 10.5 microM hemin under maximal velocity conditions. The inhibition was not prevented by dithiothreitol or glutathione, suggesting that the hemin was not interacting with the sulphydryl groups of ferrochelatase. Human serum albumin, but not bovine serum albumin was able to prevent the inhibition consistent with the known formation of the tightly bound methemalbumin complex with human albumin. Kinetic studies performed under initial velocity conditions with hemin concentrations ranging from 2 to 8 microM revealed the inhibition to be non-competitive with respect to the metal substrate (zinc) and competitive with respect to the porphyrin substrate (mesoporphyrin). The kinetic analysis indicated that hemin binds to both the enzyme and enzyme-metal complex at a site normally occupied by the porphyrin substrate, and a second molecule of hemin could bind to the enzyme-metal complex but with a much lower affinity than the first molecule. We conclude that the product inhibition of ferrochelatase by hemin should be considered as a possible site of regulation of heme biosynthesis.