THIOL METHYLTRANSFERASE IN HUMANS - DEVELOPMENT AND TISSUE DISTRIBUTION

Thiol methyltransferase (EC 2.1.1.9, TMT) activity was measured with 2 -mercaptoethanol in the microsomal fraction of 12 placenta and 31 feta l and 33 adult liver specimens. TMT activity (nmol/min incubation/mg p rotein; mean +/- SD) was 0.61 +/- 0.25 (placenta), 0.74 +/- 0.45 (feta l liver), and 4.51 +/- 2.29 (adult liver). TMT activity was also measu red in extrahepatic tissues and it was about one order of magnitude lo wer in fetal lungs, kidney and intestine as compared with the fetal li ver. A similar distribution pattern was also observed in adult tissues except that in the kidney TMT activity was one third of the hepatic o ne. Studies of enzyme kinetics showed that fetal and adult hepatic TMT obeyed non-Michaelis-Menten kinetics when 2-mercaptoethanol was the v arying substrate. Average values of K(m) for the higher and lower affi nity phases were 0.03 and 14.05 mmol/l, respectively (fetal liver) and 0.005 and 14.57 mmol/l, respectively (adult liver). This paper shows that TMT develops prenatally and its distribution pattern is consisten t with that of other microsomal enzymes, being preferentially associat ed with the liver both in the human fetus and in adult subject.