Muscle-specific regulation of the heme biosynthetic enzyme 5'-aminolevulinate synthase.

The induction of 5'-aminolevulinate synthase (ALV synthase) activity in adult muscle by overload occurs in the absence of proportional changes in its mRNA content. Complete interpretation of these findings is difficult because little is known of the basal regulation of ALV synthase expression in muscle. In three adult chicken muscle fiber types (n = 5 each), differences in ALV synthase activity were correlated (r greater than or equal to 0.89; P less than 0.05) to the activities of cytochrome oxidase (COX) and citrate synthase (CS) and to levels of the "liver" isoform of ALV synthase mRNA. During posthatch development, ALV synthase activity and mRNA levels (n = 3-6 per time point) also covaried with changes in COX and CS activity. The highest levels of ALV synthase mRNA in muscle are observed early in myogenesis prior to induction of COX activity. The regulation of ALV synthase is also tissue-specific because the higher basal levels of ALV synthase activity in liver mitochondria are associated with disproportionately less oxidative enzyme activity and less of the liver ALV synthase isoform mRNA than in muscle.