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The LH biological potency of the International Reference Preparation (IRP) of Human Pituitary LH for Immunoassay (IRP 68/40) relative to that of the 2nd IRP of Human Pituitary FSH and LH for Bioassay (IRP 78/549) is markedly greater when estimated by in-vitro interstitial cell testosterone production (TICT) bioassay than by in-vivo bioassay, and by the 4-h ovarian ascorbate depletion (OAAD) assay than by the 4-day seminal vesicle weight gain assay. Other preparations of human LH which, like IRP 68/40, were highly purified, showed a similar spectrum of bioactivity in these assay systems and also contained a higher proportion of more basic LH isoforms than are found in crude pituitary extracts such as IRP 78/549.

In an attempt to explain these differences, a comparison was made of the plasma survival in rats of the LH bioactivity (by TICT assay) of these two preparations. Contrary to expectation, their relative plasma clearance rates over a 4-h period did not account for their differing bioactivities. The plasma half-life of the LH bioactivity (with 95% confidence limits) was estimated to be 42·4 (35·3–49·5) min for IRP 68/40 and 41·3 (31·5–51·0) min for IRP 78/549.

Furthermore the time-course of action in vivo of IRP 78/549 did not appear to be more prolonged than that of IRP 68/40. Thus their plasma testosterone responses during the course of these 4-h plasma clearance studies were similar, and estimates of the LH potency of IRP 68/40 relative to that of IRP 78/549 were no greater by 2-h than by 4-h OAAD assay.

The more basic isoforms of LH present in IRP 68/40 and in other purified human LH preparations may therefore differ from those in crude pituitary extracts, such as IRP 78/549, in their intrinsic activity to stimulate the different target cells in these assay systems rather than in their bioavailability in vivo.

J. Endocr. (1988) 119, 327–334

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