Discrepancies in plasma LH activities as measured by radioimmunoassay and an in vitro bioassay.

Abstract

ABSTRACT Using a highly sensitive in vitro bioassay system, luteinizing hormone activity has been measured in parallel with radioimmunoassays in postmenopausal plasma and plasma obtained from women in various phases of the menstrual cycle (follicular, mid-cycle, luteal). Biological and immunological activities were measured directly in plasma samples without any chemical manipulation. The biological activity (B) was always higher than the immunological activity (I); the B/I ratio varied from 2.1 to 14.0. Gel filtration of pooled plasma samples through Sephadex G-100 revealed major discrepancies in each physiological state when immunological and biological activities were measured in each fraction. The biological activity was eluted as a single peak behind the elution volume of bovine serum albumin, but in front of the elution volume of chymotrypsinogen. It was invariably preceded by a small hump. The immunological activity was spread all over the chromatogram. Areas of immunological activity without any biological activity were located on either side of the biologically active fractions, both in the high molecular weight range (including the void volume) and in the low molecular weight range. The biological LH activity recovered following fractionation on Sephadex G-100 was in close agreement with that loaded, whereas the immunological activity recovered following gel filtration exceeded the loaded activity by a factor of 6–8. In the various physiological states, 11 to 44 % of the total immunological activity recovered was not associated with any biological activity. Furthermore, there was a marked variation in the ratio of biological to immunological activities of those fractions which contained biological activity. It is suggested that the specificity of current RIA methods could be improved significantly by preparing antisera which react only with biologically active LH species.