IMMUNOLOGICAL ACTIVITIES OF HIGHLY PURIFIED ISOFORMS OF HUMAN FSH CORRELATE WITH IN-VITRO BIOACTIVITIES

In a recent study, a five- to eightfold range in human FSH radioreceptor activity (RRA) was documented for highly purified isoforms of FSH when the data were expressed on an FSH protein content basis as determined by amino acid analysis. This study examined the FSH in vitro bioactivity and immunoactivity of these preparations. FSH in vitro biological activity showed a five- to eightfold range in activity with a high correlation with the RRA values (r=0.82). A similar five- to eightfold range of values was obtained with a specific FSH radioimmunoassay and an FSH two-site immunoassay with high correlations again observed between each other, between each immunoassay and with either the in vitro bioassay or the RRA method (r=0.77-0.995). Although there was overall a close correlation between these assays, significant differences in ratios of activities between the in vitro bioassay and other methods were observed with highly purified FSH isoform preparations from different pI regions. The high correlation between in vitro bioassay/RRA methods and immunoassay methods over a wide range of isoform specific activities suggests that these methods are detecting similar structural features on each isoform. It is thus concluded that these immunoassays are not solely measuring hormone mass based entirely on amino acid composition. This conclusion raises questions about ratio measurements of FSH, where immunoassay methods are presumed to measure total protein content, and their application in physiological situations and clinical practice.