Characterization and developmental regulation of tyramine-β-hydroxylase in the CNS of the moth, Manduca sexta

Abstract

Octopamine (OA) is present in insect nervous tissue, but little is known about its biosynthesis. In the CNS of Manduca sexta, OA levels increase markedly during postembryonic adult development. To study this increase, we developed an assay for tyramine-β-hydroxylase, the putatively rate-limiting enzyme for OA biosynthesis. Tyramine-β-hydroxylase activity in extracts of M. sexta CNS tissue: (1) was time- and protein-dependent, and with protein concentrations up to 2 μg/μl, was linear for 20 min; (2) had a pH optimum of 7.0 for conversion of tyramine to OA; (3) required ascorbate, copper, and catalase; and (4) had an apparent K M, tyramine of 0.22±0.04 mM. These characteristics resemble those of the mammalian enzyme dopamine-β-hydroxylase, suggesting that these two enzymes are functionally related. During adult development, tyramine-β-hydroxylase activity increased 11-fold in the brain and 9-fold in the abdominal ganglia, paralleling increases in OA levels in those CNS structures during metamorphosis. The apparent kinetic constants of tyramine-β-hydroxylase suggested that the amount of this enzyme present in the tissues increases. The increase in OA levels during adult development thus appears to be due to an increase in the level of enzyme available for OA synthesis and may reflect an increase in the number of octopaminergic neurons.