Diurnal variation and melatonin induction of hepatic molybdenum hydroxylase activity in the guinea-pig

The activities of the xenobiotic metabolizing enzymes, aldehyde oxidase and xanthine oxidase, were determined in partially purified fractions of adult guinea-pig liver at given times in the day or night. A marked circadian variation in aldehyde oxidase activity was observed with several substrates (phthalazine, phenanthridine, N-phenylquinolinium and 3, 4-dihydro-4-hydroxy-3-methyl-2-quina-zolinone). The main peak occurred at 0300 hr with minimum activity from 1200 to 1800 hr, the differences between rhythmic extremes being statistically significant (P < 0.005). Xanthine oxidase activity also exhibited a daily rhythm but with a lower amplitude. Guinea-pig serum melatonin showed a synchronous circadian fluctuation with peak values at 0300 hr falling throughout the day to a minimum at 1800 hr. Exogenously administered melatonin caused a significant increase in aldehyde oxidase activity at 0900 and 1200 hr and in xanthine oxidase activity at 0900 hr. It was concluded that melatonin concentrations may be related to the circadian variation in liver molybdenum hydroxylase activity.