Endocrine Abstracts

Macroprolactin is a high molecular mass prolactin isoform found in some normal individuals. The most common form of macroprolactin arises from binding of prolactin to IgG ('big-big' prolactin), with a molecular mass 150-170 kD. A further prolactin isoform of intermediate size ('big' prolactin, 50-60 kD), has also been described but the nature and significance of this form is less clearly understood.

We have previously described the effects of pregnancy in hyperprolactinaemia due to big-big prolactin, reporting a rise in normal prolactin but a progressive fall in big-big prolactin (in parallel with a fall in IgG). We now report a patient with hyperprolactinaemia due to both big and big-big prolactin, and describe the effects of pregnancy on these prolactin forms. Serum was obtained before pregnancy and at 16, 24 and 28 weeks' gestation. Prolactin was measured by electrochemiluminescent assay (Roche Elecsys). Initial macroprolactin screening was by PEG precipitation, and the relative contribution of macroprolactin species was quantified by gel filtration chromatography. Before pregnancy total serum prolactin was elevated at 3990 mU/l, of which 93% was macroprolactin. During pregnancy there was a 10-fold rise in total prolactin; in contrast to the results of our previous study, in this case there was a progressive rise in the contribution from high molecular mass components of serum PRL. Chromatography demonstrated the presence of both big-big and big prolactin, with big prolactin being a substantial component of total prolactin immunoactivity.

These results demonstrate contrasting effects of pregnancy on different prolactin isoforms and show that big prolactin differs from big-big prolactin not only in size but also in the changes seen during pregnancy. This study also provides further evidence for heterogeneity of prolactin species. Better understanding of the molecular nature of big prolactin may provide an explanation for the differential effects of pregnancy on different prolactin isoforms.