Studies on liver phosphatidyl cholines: II. Effect of sex, age and species differences on phosphatidyl cholines from liver mitochondria and microsomes

Microsomal phosphatidylethanolamine methyltransferase: inhibition by S-adenosylhomocysteine

Inhibition by S-adenosylhomocysteine (AdoHcy) of the three reactions of phosphatidylethanolamine methyltransferase which catalyzes the production of phosphatidylcholine from phosphatidylethanolamine in guinea pig and rat liver microsomes has been evaluated. Five of the six methylation reactions in these two species exhibit greater affinity for inhibitor, AdoHcy, than for substrate, S-adenosylmethionine (AdoMet). The Ki values for the rate-limiting reactions were 3.8 microM and 68 microM in rat and guinea pig livers, respectively. An AdoMet : AdoHcy ratio of 12 : 1 in developing liver was found to decline to a constant value in the adult of 5 : 1. The concentration of AdoHcy in rat and guinea pig liver increases markedly following death of the animal. A concomitant decrease in the AdoMet level was observed in guinea pig liver. A comparison of phosphatidylethanolamine methyltransferase activity with the hepatic concentrations of AdoMet and AdoHcy in mouse, rat, rabbit and guinea pig is presented. Regulation of the methylation pathway is discussed.

Microsomal phosphatidylethanolamine methyltransferase: some physical and kinetic properties

Some physical and kinetic properties of the microsomal enzyme(s) that convert phosphatidylethanolamine to phosphatidylcholine in rat and guinea pig livers have been investigated. The pH optima of the reactions were 9.8, 9.3 and 9.5 for the first, second and third methylation reactions, respectively. Incomplete heat denaturation of the protein catalyzing the first reaction contrasts with inactivation at 60 C of the enzymes catalyzing the second and third methylations. The maximal velocity of the first reaction of the guinea pig liver enzyme is 48 pmol/min/mg protein, substantially less than exhibited rate-limiting reaction of the three step methylation sequence in rat liver, 114 pmol/min/mg. The affinity of the microsomal enzyme for S-adenosylmethionine is greater in rat liver (Km = 18.2 microM) than in guinea pig liver (Km = 302 microM).