Analysis of fluctuations in the activity of rat pineal tyrosine hydroxylase

Tyrosine hydroxylase was measured in pineals removed from light-adapted and dark-adapted rats. The animals were adapted to the appropriate light situation for approximately 5 h before removal of the pineals. Bilateral ganglionectomy confirmed that all detectable enzyme activity in the pineal appears present in sympathetic fibers originating in the superior cervical ganglia. An analysis of tyrosine hydroxylase at subsaturating cofactor concentrations in dark-adapted rats revealed activity which was 60% higher than in appropriate light control animals. Further kinetic analysis showed that the K(m) for the synthetic cofactor, 6-methyltetrahydropterin, for dark-adapted animals was 0.8 + or - 0.1 mM which was significantly lower than a K(m) value of 1.4 + or - 0.2 mM for light adapted animals. Thus the rate of norepinephrine synthesis in vivo could increase due to an alteration in the enzyme molecule, which affects its interaction with substrate. These findings support previously documented results which show both an increased turnover and an increase in the concentration of norepinephrine in the pineals of dark-adapted rats.