Circulating macroprolactin exhibits molecular heterogeneity and is not exclusively an antibody complex

Background: Macroprolactin (macPRL) is considered to be solely a prolactin antibody complex. We examined macPRL heterogeneity in samples from thirteen patients suspected of macroprolactinemia. Methods: Polyethylene glycol (PEG) precipitation, gel permeation (GPC), protein-G affinity, and Lectin affinity chromatography were used to investigate the nature of macPRL. Results: Using PEG, 8, 3, and 2 samples were macPRL positive, negative, and indeterminate respectively. Using GPC, prolactin appeared at high (H) (>= 150 kDa), mid (M) (>= 30 <150 kDa), and low (L) (<30 k Da) forms. For macPRL positive samples, 52.3 to 95.0%, 3.6 to 34.1%, and 1.4 to 34.5% appeared at the (H), (M), and (L) regions respectively, compared with samples negative for macPRL with 1.2 to 5.1%, 60.0 to 79.4%, and 15.4 to 38.9% prolactin activity respectively. macPRL positive samples showed 30.4 to 86.5% binding to protein G column compared with negative samples at 1.2 to 5.1%. GPC-separated forms showed macPRL is heterogenous being either antibody bound (protein G studies) or glycosylated aggregates (lectin studies). Samples with identified macPRL forms were analysed using 4 immunoassay analysers. Conclusions: Samples with (H) and (M) macPRL forms showed significant positive bias in 2 immunoassays. The study is limited by the small number of samples and a larger scale study is required.

Automated summary

Macroprolactin (macPRL) was found to exhibit heterogeneity in samples, possibly due to factors such as antibody binding or glycosylated aggregates. Samples with (H) and (M) macPRL forms showed significant positive bias in two of the four immunoassay analysers used. Further studies on a larger scale are required due to the limited sample size.