Heterogeneity of Pituitary and Plasma Prolactin in Man: Decreased Affinity of “Big” Prolactin in a Radioreceptor Assay and Evidence for Its Secretion*

Abstract

Molecular heterogeneity of immunoreactive human PRL (IR-hPRL) plasma was assessed by exclusion chromatography on Sephadex G-100 in blood drawn from 4 normal adults, 3 newborn infants, 2 late gestational women, 3 patients with primary hypothyroidism and high PRL levels, 2 with functional hyperprolactinemia, 3 with acromegaly, and 10 with PRLsecreting tumors. Three forms of PRL, each of different apparent molecular weight, were detected: the so-called big-big hPRL eluted with the void volume, big hPRL appeared after the albumin peak, and little hPRL eluted as monomeric hPRL. In normal subjects, the proportion of big-big, big, and little hPRL components was 5.1 ± 1.7%, 9.1 ± 0.9%, and 85.8 ± 2.3%, respectively, without significant change in the distribution of the different forms after TRF stimulation. In 8 of 10 patients with PRL-secreting tumors, we detected 3.9 ± 0.8%, 14.3 ± 1.1%, and 81.8 ± 1.1%, respectively, a significantly higher proportion of big PRL. In 2 additional patients with prolactinomas, the proportion of big PRL was much higher. In 3 of 10 patients, the molecular heterogeneity of the tumor PRL was similar to that in plasma. In 1 acromegalic, there was a very high proportion of big-big hPRL, the molecular heterogeneity of which differed from that of hGH. The different PRL fractions were tested in a radioreceptor assay (RRA) using membranes from rabbit mammary gland; the receptor-binding activity was compared with immunoactivity by using the ratio of receptor activity to immunoactivity and was designated as the PRL-binding index. Big PRL was much less active than little PRL in the RRA with a mean PRL-binding index of 0.27 ± 0.05, in contrast to 1.16 ± 0.07 for little PRL. The fractions were rechromatographed after storage. Big PRL partially distributed as little or big-big PRL, while little PRL remained unchanged. This conversion was weakly related to the time of storage and was accelerated by repeated freezing-thawing cycles or guanidine treatment. Big-big PRL from tumor extract partially converted into big and little PRL. As in the original fraction, the big PRL obtained by rechromatography after storage had low activity in the RRA; also, after repeated freezing-thawing cycles or guanidine treatment, it exhibited nonparallelism. These observations suggest that at least part of the receptor activity of the big PRL fraction may arise from generation of or contamination by little PRL. The decreased binding affinity of big PRL in the RRA also indicates a lower affinity of big PRL for mammary membranes, which are likely to contain specific receptors for lactogenic hormones, suggesting that big PRL has little, if any, biological activity. The evidence, while incomplete, suggests that big PRL is a native PRL dimer linked by intermolecular disulfide bonds which arises in the lactotrope as a postsynthetic product or derivative and is not a true precursor prohormone.