Conversion of spironolactone to 7α-thiomethylspironolactone by hepatic and renal microsomes

Abstract

Recent observations indicate that 7α-thiomethylspironolactone is an important circulating metabolite of the mineralocorticoid antagonist spironolactone (SL). Studies were carried out to determine possible sites and pathways of 7α-thiomethyl-SL formation and, in particular, to evaluate SL metabolism by guinea pig hepatic and renal microsomal preparations. In the absence of S-adenosylmethionine (SAM), liver and kidney microsomes rapidly converted SL to 7α-thio-SL as the only metabolite. The rate of 7α-thio-SL production was greater in liver than kidney. In the presence of SAM, 7α-thio-SL was further converted to 7α-thiomethyl-SL by liver and kidney microsomes. The rates of methylation with 7α-thio-SL as substrate were three to four times greater for liver than for kidney, but the K m values were similar (~30 μM) in the two tissues. Maximal enzyme activity was obtained with SAM concentrations of 25–200 μM. NADPH had no effect on SL or 7α-thio-SL metabolism by liver or kidney microsomes. To determine if a pathway involving the C-S lyase enzyme might contribute to circulating 7α-thiomethyl-SL levels in vivo, guinea pigs were treated with SL or its dethioacetylated derivative, canrenone, and plasma metabolites were analyzed by HPLC. Both 7α-thiomethyl-SL and canrenone were found to be circulating metabolites in SL-treated animals, but only canrenone was identified in the plasma of canrenone-treated guinea pigs. The results indicate that the liver and kidney are potential sites of 7α-thiomethyl-SL production and that its formation probably does not involve the C-S lyase pathway.