Isolation of prolactin from salmon pituitary

Prolactin(s) was isolated from salmon pituitary by chromatography on concanavalin A, gel filtration, and ion-exchange chromatography. The prolactin not adsorbed on DEAE Bio-Gel A (DEAE 1) had the same molecular weight by get filtration before and after ion-exchange chromatography. This salmon prolactin had a molecular weight of 24,300 by gel filtration and 20,500 by SDS gel electrophoresis. The isoelectric point was 6.0 when determined by isoelectric focusing. This prolactin was homogeneous by electrophoresis under acidic and basic conditions and essentially homogeneous by isoelectric focusing. The protein consistently maintained plasma sodium levels in hypophysectomized Poecilia latipinna at very low dosage (50 ng/fish). Antibodies against this prolactin localized specifically in the prolactin cells of rainbow trout pituitary. A protein fraction more strongly adsorbed on DEAE Bio-Gel A (DEAE 4) also had some prolactin activity in the Poecilia assay. Prior to DEAE chromatography this protein occurred in the same molecular weight fraction as the nonadsorbed protein but after DEAE chromatography the fraction contained three proteins with molecular weights of 23,000 (64%), 46,000 (21%), and 66,000 (15%). The results suggest that the higher molecular weight proteins are aggregates formed during isolation but the 23,000 molecular weight protein may be a modified form of the unadsorbed prolactin. The growth hormone fraction (DEAE 2) had no prolactin activity when tested at 10 times the minimum effective dose of salmon prolactin. The remaining DEAE fractions, 3 and 5, were also inactive in the Poecilia assay.