Fatty Acid Synthetase from Lactating Rat Mammary Gland: III. DISSOCIATION AND REASSOCIATION

Abstract

Abstract Fatty acid synthetase from lactating rat mammary gland is shown to be a cold-labile multienzyme complex. The native (13 S) form slowly dissociates into half-molecular weight (9 S) subunits on aging in the cold. Fatty acid synthetase activity observed on addition of the 9 S subunits to the assay system between 20 and 30°, is shown to result from rapid reassociation of the subunits to the 13 S form. Dissociation of the enzyme into subunits is accompanied by a change in the number of protein sulfhydryl groups accessible to 5,5'-dithiobis(2-nitrobenzoate), but reassociation to the parent form is not dependent on the presence of a reduced thiol. The dissociation does not involve oxidation of protein sulfhydryl groups, as the total number of sulfhydryl groups titratable with 5,5'-dithiobis(2-nitrobenzoate) in the presence of 6 m urea was the same in the native enzyme and its subunits. Blocking more than two of the 28 subunit sulfhydryl groups with p-chloromercuribenzoate restricted the ability of the subunits to undergo heat-induced reassociation. Evidence is presented which suggests that although protein sulfhydryl groups may be involved in the dissociation-reassociation phenomenon, hydrophobic bonding is probably a more critical factor. Similar studies carried out with the fatty acid synthetase purified from rat liver indicate that this enzyme too exhibits cold lability.