Uptake and processing of prolactin by alternative pathways in rat liver

The uptake and subcellular processing of radiolabelled prolactin has been studied in male and female rats. Analytical subcellular fractionation of liver homogenates from rats injected with 125I-prolactin showed that in female rats the prolactin was primarily internalised to low density (1.12 g X cm-3) membranes. Approx. 10-15% of the total label was found in high density membranes, similar in distribution to lysosomal marker enzymes. In the normal male rat, prolactin was internalised solely to lysosomal type membranes. However, in male rats treated with estrogen, the distribution of prolactin was very similar to that seem in the female, indicating that internalisation to low density membrane is dependent on the presence of prolactin receptors. Gel exclusion chromatography showed that the prolactin internalised to the lysosomal membranes was extensively degraded whereas that associated with the low density membrane remained intact. Use of digitonin, to establish the identity of the low density membrane gave inconclusive results, but suggested that the prolactin was associated with membrane bearing NADH pyrophosphatase rather than the classical Golgi marker, galactosyltransferase.