Regulation of microsomal stearoyl-coenzyme A desaturase. Purification of a non-substrate-binding protein that stimulates activity

Crude cytosolic fraction from rat liver was examined for proteins that may be involved in regulation of microsomal stearoyl-CoA desaturase activity. Gel filtration revealed the presence of several components that either stimulate or inhibit this enzyme. In addition, other components bind the acyl-CoA substrate, thus affecting observed activities in vitro. A protein that stimulates stearoyl-CoA desaturase but does not bind substrate was purified approx. 1100-fold. The purified protein had no visible absorption spectrum and an approximate mol.wt. of 26500. Maximal stimulation of desaturase activity occurred with less than 2 micrometer purified protein. The protein was heat-labile and exhibited neither catalase nor glutathione peroxidase activity. Addition of the cytosolic protein produced no effect on the desaturase reaction stoicheiometry; the proportions O2 consumed/NADH oxidized/stearoyl-CoA desaturated remained 1:1:1. Because the Km' for stearoyl-CoA was also unchanged by addition of the cytosolic protein, no change in substrate affinity was suggested. Furthermore addition of the cytosolic protein also produced no effect on desaturase inhibition by oleoyl-CoA, which suggested that the protein does not simply relieve apparent product inhibition. These results indicate that, in analogy to other cytosolic proteins that stimulate microsomal oxidase activities, the protein may have a regulatory function, perhaps related to activity modulation via organization of the multienzymic desaturase in the membrane.