COMPARATIVE METABOLISM OF ISOLEUCINE BY CORPORA ALLATA OF NONLEPIDOPTERAN INSECTS VERSUS LEPIDOPTERAN INSECTS, IN RELATION TO JUVENILE-HORMONE BIOSYNTHESIS

We studied the metabolism of [U-C-14]isoleucine by intact and homogenized corpora allata (CA) from various insect species to determine how this substrate is converted to precursors of juvenile hormone (JH). CA homogenates of the lepidopterans Manduca sexta, Hyalophora cecropia, and Samia cynthia metabolize [U-C-14]isoleucine to several products including 2-keto-3-methylvalerate, 2-methylbutyrate, CO2, propionate, and acetate. Intact CA of male H. cecropia produce particularly high levels of 2-keto-3-methylvalerate, indicating a highly active branched-chain-amino acid transaminase. In contrast, CA homogenates from the nonlepidopterans Periplaneta americana, Schistocerca nitens, Tenebrio molitor, and Diploptera punctata barely metabolize [U-C-14]isoleucine. However, P. americana CA homogenate metabolizes [U-C-14]2-keto-3-methylvalerate, the transamination product of [U-C-14]isoleucine, more rapidly than does a homogenate of M. sexta CA. Furthermore, intact CA from P. americana incubated with [U-C-14]2-keto-3-methylvalerate incorporate low levels of C-14 into JH III, but do not metabolize this substrate to JH II or JH I. Intact CA from female Diploptera punctata produce very high levels of JH III, but are also unable to incorporate radiolabel from [U-C-14]isoleucine into JH III, which substantiates our findings with other nonlepidopteran CA. The results suggest that CA of nonlepidopteran insects lack an active branched-chain amino acid transaminase and, consequently, are unable to utilize these substrates for JH biosynthesis.