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The synthesis and maturation of the precursor forms of three mitochondrial enzymes involved in steroid hormone biosynthesis have been studied in vivo. Primary cultures of bovine adrenocortical cells were radiolabeled with [35S] methionine and newly synthesized cholesterol side-chain cleavage cytochrome P-450 (P-450scc), 11 beta-hydroxylase cytochrome P-450 (P-450(11)beta), and adrenodoxin immunoisolated using specific antibodies. Both the precursor and mature forms of P-450scc and P-450(11)beta were detected during short periods of pulse labeling; however, the precursor forms were transitory in nature while their corresponding mature forms accumulated. Pulse-chase experiments showed that the precursor form of each cytochrome P-450 had an apparent half-life of 3.5 min. In contrast, the precursor form of adrenodoxin was not readily detected in pulse-labeling experiments until a substantial amount of its mature form had accumulated. When the cultured cells were treated with a chelator of divalent cations (o-phenanthroline) or a mitochondrial uncoupler (dinitrophenol), the maturation of all three precursors was inhibited. The synthesis of the P-450scc and P-450(11)beta precursors was induced in cells maintained in the presence of adrenocorticotropin, and the rates of appearance of their processed forms were also increased. The mature forms of all three proteins were immunoisolated from a trypsinized mitochondrial fraction prepared from the radiolabeled cells, demonstrating that the mature proteins were localized within the organelle. These studies establish that the maturation of the precursor forms of the mitochondrial steroidogenic enzymes are characterized by steps similar to those reported for other mitochondrial precursor proteins.