The metabolism of a typical type-I substrate, aminopyrine (58151), by microsomes prepared from the skin of neonatal rats was reported. Skin microsomes were prepared from Sprague-Dawley-rats on day four after birth. The neonatal rat skin microsomes were incubated with carbon-14 labeled aminopyrine for 20 minutes, and then the quantity of formaldehyde formed was determined. The presence of aminopyrine- N-demethylase activity in the microsomes was demonstrated. The drug metabolizing enzyme activity was NADPH dependent. Activity in the presence of NADH alone was only 37 percent that of NADPH. Addition of NADH to incubation mixtures containing NADPH resulted in an 8 percent increase in formaldehyde formation. No formaldehyde formation was noted when either boiled enzyme was used in the complete incubation system or when the pyridine nucleotide or the regenerating system was omitted from the incubation mixture. Cofactor requirements for neonatal skin aminopyrine-N-demethylation closely resembled those of the hepatic N-demethylase. Under the conditions of this aminopyrine-N-demethylase assay, formaldehyde formation by neonatal rat skin microsomes was directly proportional to the amount of microsomal protein employed and to the time of incubation. The data indicated that skin does contain aminopyrine-N- demethylase activity and that skin microsomes are active in metabolizing a type-I substrate such as aminopyrine. These data emphasized that the small amount of measurable-P-450 in skin microsomes is capable of considerable metabolism of appropriate substrates.
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