The existence of an inner-membrane-bound, long acyl-chain-specific 3-hydroxyacyl-CoA dehydrogenase in mammalian mitochondria.

L-3-Hydroxyacyl-CoA dehydrogenase was measured in mammalian tissues with long-chain (C16) and short-chain (C4) 3-ketoacyl-CoA substrates. The majority of the activity was mitochondrial. Activity with C16 substrate was enriched in the inner membrane fraction while the matrix showed enrichment of activity with C4 substrate. Gel-filtration in the presence of nonionic and ionic detergents separated two peaks of activity with C16 substrate from liver, heart, kidney and brown adipose tissue mitochondria. The highest molecular weight activity was associated with membrane fragments and accounted for 18-32% of the total activity with C16 substrate. This enzyme showed preference for long-chain substrate, the C16/C4 activity ratio being 6-15 for the different mitochondria studied. The other enzyme had a molecular weight of 71 000 and showed highest activity with C4 substrate (C16/C4 activity ratio of 0.2). The substrate specificity of the membrane-associated enzyme was not altered by extraction with detergent or lipid solvents. The membrane-associated enzyme could be partially purified by treatment with high concentrations of KCl and EDTA, when it showed a molecular weight of 186 000 while retaining its high C16/C4 activity ratio. Protease treatment caused loss of C4 activity from the high-molecular-weight enzyme without affecting its C16 activity. We conclude that an inner membrane-associated, long-chain-specific 3-ketoacyl-CoA dehydrogenase coexists in mitochondria with the better-documented matrix enzyme which shows preference for shorter-chain-length substrates.