Serum prolactin concentrations were measured using a sensitive bioassay (Nb2 assay) and by radioimmunoassay in 11 patients with prolactin-secreting pituitary tumours (median serum immunoactive prolactin 5150 mU/L), and in 58 normal control subjects (median prolactin 190 mU/L). The mean ratio of serum prolactin bioactivity to immunoactivity was significantly lower in patients with prolactinomas than in normoprolactinaemic controls. Ten lactating women in the early post-partum period (median prolactin 3800 mU/L), studied as a model of physiological hyperprolactinaemia, also had reduced bioactivity to immunoactivity ratios. Overall, there was a significant negative correlation between bioactivity : immunoactivity ratio and serum immunoactive prolactin.
The change in relative bioactivity of prolactin in serum samples from patients with prolactinomas and in women with physiological hyperprolactinaemia may reflect changes in the molecular heterogeneity of the hormone. Such changes may affect activity in both bioassays and radioimmunoassays.
LewisUJSinhaYNMarkoffEVanderlaanWP. Multiple forms of prolactin: Properties and measurement. In: MullerEEMacLeodRMFrohmanLA, eds. Neuroendocrine Perspectives, vol. 4. Amsterdam: Elsevier Scientific, 1985: 43–57.
2.
LewisUJSinghRNPSinhaYNVanderlaanWP. Glycosylated human prolactin. Endocrinology1985; 116: 359–63.
3.
GarnierPEAubertMLKaplanSLGrumbachMM. Heterogeneity of pituitary and plasma prolactin in man: Decreased affinity of ‘big’ prolactin in a radioreceptor assay and evidence for its secretion. J Clin Endocrinol Metab1978; 47: 1273–81.
4.
FarkouhNHPackerMGFrantzAG. Large molecular size prolactin with reduced receptor activity in human serum: High proportion in basal state and reduction after thyrotropin-releasing hormone. J Clin Endocrinol Metab1979; 48: 1026–32.
5.
JacksonRDWortsmanJMalarkeyWB. Characterisation of a large molecular weight prolactin in women with idiopathic hyperprolactinaemia and normal menses. J Clin Endocrinol Metab1985; 61: 258–64.
6.
MarkoffESigelMBLacourNSeaveyBKFriesenHGLewisUJ. Glycosylation selectively alters the biological activity of prolactin. Endocrinology1988; 123: 1303–6.
7.
PellegriniIGunzGRoninCPolymorphism of prolactin secreted by human prolactinoma cells: Immunological, receptor-binding and biological properties of the glycosylated and non-glycosylated forms. Endocrinology1988; 122: 2667–74.
8.
TanakaTShiuRPCGoutPCA new sensitive and specific bioassay for lactogenic hormones: Measurement of prolactin and growth hormone in human serum. J Clin Endocrinol Metab1980; 51: 1058–63.
9.
SmithCRButlerJNormanMR. Biological activity of serum prolactin in patients with primary hypothyroidism. Ann Clin Biochem1988; 25540–5.
10.
RoweRCCowdenEAFaimanCFriesenHG. Correlation of Nb2 bioassay and radioimmunoassay values for human serum prolactin. J Clin Endocrinol Metab1983; 57: 942–6.
11.
PavlouCChardTLetchworthAT. Circulating levels of human chorionic somatomammotrophin in late pregnancy: Disappearance from the circulation after delivery, variation during labour and circadian variation. J Obstet Gynecol1972; 79: 629–34.
12.
HutchinsonJSM. The interpretation of pituitary gonadotrophin assays—a continuing challenge. J Endocrinol1988; 118: 169–71.
13.
RogolADRosenSW. Prolactin of apparent large molecular size: The major immunoactive prolactin component in a patient with a pituitary tumour. J Clin Endocrinol Metab1974; 38: 714–7.
14.
BenvenisteRHelmanJDOrthDNMcKennaTJNicholsonWERabinowitzD.Circulating big human prolactin: Conversion to small human prolactin by reduction of disulphide bonds. J Clin Endocrinol Metab1979; 48: 883–6.
15.
SuhHKFranzAG. Size heterogeneity of human prolactin in plasma and pituitary extracts. J Clin Endocrinol Metab1974; 39: 928–35.
16.
GuydaHJ. Heterogeneity of human growth hormone and prolactin secreted in vitro: Immunoassay and radio-receptor assay correlations. J Clin Endocrinol Metab1975; 41: 953–67.
17.
WhitakerMDKleeCGKaoPCRandallRVHeserDW. Demonstration of biological activity of prolactin molecular weight variants in human sera. J Clin Endocrinol Metab1984; 58: 826–30.
18.
MarkoffELeeDW. Glycosylated prolactin is a major circulating variant in human serum. J Clin Endocrinol Metab1987; 65: 1102–6.
19.
HashimIAstonRButlerJSmithCRNormanMR. Molecular heterogeneity of prolactin in serum from normal individuals. Ann Clin Biochem1988; 25 (Suppl.): 125s.
20.
HashimIAstonRButlerJMcGregorAMSmithCRNormanMR. Decreased proportion of glycosylated prolactin in the serum of patients with suspected prolactinomas. J Endocrinol1988; 117: (Suppl.): abstract 88.
21.
WhittakerPGWilcoxTLindT.Maintained fertility in a patient with hyperprolactinaemia due to big, big prolactin. J Clin Endocrinol Metab1981; 53: 863–6.
22.
LarreaFVillanuevaCCraniotoMCEscorzaAdel RealO.Further evidence that big-big prolactin is preferentially secreted in women with hyper-prolactinaemia and normal ovarian function. Fertil Steril1985; 44: 25–30.
23.
SubramanianMGSpirtosNJMoghissiKSMagyarDMHayesMFGalaRR. Correlation and comparison of Nb2 lymphoma cell bioassay with radioimmunoassay for human prolactin. Fertil Steril1984; 42: 870–4.
24.
TanakaTShishibaYGoutPWBeerCTNobleRLFriesenHG. Radioimmunoassay and bioassay of human growth hormone and human prolactin. J Clin Endocrinol Metab1983; 56: 18–20.
25.
TomitaKMcCoshenJAFriesenHGTysonJE. Quantitative comparison between biological and immunological activities of prolactin derived from human fetal and maternal sources. J Clin Endocrinol Metab1982; 55: 269–71.
26.
McNeillyASFriesenHG. Presence of a nonlactogenic factor in human serum which synergistically enhances prolactin-stimulated growth of Nb2 rat lymphoma cells in vitro. J Clin Endocrinol Metab1985; 61: 408–11.
