Monitoring of the specific radioactivity of S-adenosylmethionine in kidney in vivo.

The specific radioactivity of S-adenosylmethionine was followed in the cat kidney during the infusion of L-[Me-3H]methionine into the corresponding renal artery. For this purpose 14C-labelled 4-(2-aminoethyl)pyrocatechol([14C]dopamine) as methyl acceptor was injected locally every 15 min and the 3H and 14C activity of the methylation product homovanillic acid, isolated from urine, was measured. Approximately 5% of the 14C label is excreted during the first renal passage as [14C]homovanillic acid. The specific activity of S-adenosy[Me-3H]methionine in the kidney was calculated from the known specific radioactivity of [14C]dopamine injected and the measured radioactivity ratio, 3H: 14C, of homovanillic acid isolated from urine. The specific activity of S-adenosyl[Me-3H]methionine reaches a constant value in kidney about 30-60 min after the beginning of the L-[Me-3H]methionine infusion. This plateau value was 28% +/- 14% (n = 5) lower than the specific activity of L-[Me-3H]methionine in the venous blood from the corresponding kidney. The difference between the specific radioactivity of S-adenosyl[Me-3H]methionine in kidney and of free methionine in plasma is explained by the existence of a methionine source of minor specific activity in the kidney. The average life span of S-adenosylmethionine in the kidney is 19.5 +/- 8.7 min (n = 5).