Methionine adenosyltransferase and transmethylation in fetal and neonatal lung of the human, monkey, and rabbit

Optimal conditions for the assay of methionine adenosyltransferase in crude extracts of human fetal lung were determined. Maximal activity was obtained with 36 mM ATP, 20 mM L-methionine, 240 mM Mg++, and 160 mM K+. The pH optimum was 6.2--6.4, which is the same as that for adult human lung but lower than that for human liver. In human fetal lung, there was an increase in specific activity of methionine adenosyltransferase with increasing gestational age (r = 0.87; P less than 0.01) up to 25 weels of gestation, after which time no fetal specimens were obtained. The specific activity of 5-methyltetrahydrofolate-homocysteine methyltransferase of human fetal lung was 2.51 +/- 0.88 nmol/mg protein/hr, which was higher (P less than 0.001) than the activity found in newborn lung (0.14 +/- 0.01). Activity of serine hydroxymethyltransferase and of betaine-homocysteine methyltransferase was absent from human fetal lung. Activity of cystathionine synthase was absent from fetal, neonatal, and mature human lung. Activity of cystathionase in fetal and newborn human lung was present only in trace amounts. In rhesus monkey lung, beginning 15 days before term, the activity of methionine adenosyltransferase increased 6-fold to reach a maximum before term (165 days), and during the first weeks of life the activity gradually diminished. 5-Methyltetrahydrofolate-homocysteine methyltransferase activity in fetal (100--145 days) monkey lung was higher (6.57 +/- 0.95 nmol/mg protein/hr) than in newborn lungs (1.91 +/- 0.97) (P less than 0.001). In fetal rabbit lung, the activity of methionine adenosyltransferase decreased 2.5-fold during the last third of pregnancy, whereas a 2-fold increase occurred during the first 48 hr after term birth.