The nature of enzyme-substrate complexes in acyl-coenzyme A dehydrogenases

The nature of the purple complex formed upon the addition of octanoyl-CoA to the medium chain acyl-CoA dehydrogenase from pig kidney has been addressed by chemical quenching studies. Previous work, using quenching in 0.1 M KOH, suggested that the dehydrogenation product, trans-2-octenoyl-CoA, was not a participant in reduced rat liver enzyme complexes because no octenoic acid was detected after denaturation (Y. Ikeda, D. G. Hine, K. Okamura-Ikeda and K. Tanaka (1985) J. Biol. Chem. 260, 1326-1337). However, when the octanoyl-CoA-reduced pig kidney enzyme is quenched rapidly in 2 M HCl, the ratio of trans-2-octenoyl-CoA/octanoyl-CoA released is 9/1. A milder acid denaturation procedure yields the corresponding ratio of 0.4/1, i.e., now with an excess of the saturated substrate. Similarly, quenching the pig kidney dehydrogenase in 0.1 M KOH reveals only minor levels of octenoyl chains released into the supernatant. When quenching is insufficiently rapid compared to the internal equilibration of oxidized enzyme.octanoyl-CoA and reduced enzyme.octenoyl-CoA forms, the outcome is decided by the greater kinetic lability of the oxidized enzyme species. These data are fully consistent with the original ascription that the purple species observed upon reduction of the acyl-CoA dehydrogenases with substrate represents a charge transfer complex between reduced flavin as the donor and trans-2-octenoyl-CoA as the acceptor.