PubMed1984
The present studies were undertaken to explore the mechanism by which, as previous studies have shown, freeze-dried aqueous extracts (FDE) of plants of the species Lycopus virginicus and Lycopus europaeus, Melissa officinalis (Laminaceae), and Lithospermum officinale (Boraginaceae) have the ability to inhibit at least many of the effects of exogenous and endogenous TSH on the thyroid gland. To this end, we have examined the in vitro effects of FDE from these plants on the ability of bovine TSH (bTSH) to both bind to human thyroid plasma membranes (TPM) and activate adenylate cyclase therein. FDE of these four species produced a dose-related, ultimately complete, inhibition of the binding of 125I-labeled bTSH when studied at 4 C in a 20 mM Tris-HCl-0.5% BSA buffer, pH 7.45. Half-maximum inhibition of bTSH binding was produced by approximately 50 mU/ml bTSH and only about 10-30 micrograms/ml of the four active FDE. When studied in Tris-BSA-50 mM NaCl buffer at 37 C, these FDE remained inhibitory to bTSH binding, but their potency was decreased to about one fifth of that seen in the absence of NaCl. The binding of [125I]hCG to rat testis membranes was also inhibited by all of these FDE, but no effect on the binding of [125I]insulin to crude rat liver membranes was observed. In concentrations as high as 1 mg/ml, FDE of Verbena officinalis (Verbenaceae), which belongs to the same order (Tubiflorae) as the other plants, but exhibits no antithyrotropic or antigonadotropic activity in vivo, had no effect on either the binding of bTSH to thyroid membranes or the binding of hCG to rat testis membranes. No inhibition of [125I]bTSH binding occurred when TPM were preincubated with the four active FDE, washed, and then incubated with [125I]bTSH in medium devoid of FDE. Hence, the inhibition of [125I]bTSH binding seen when labeled hormone and active FDE were added together was not due to irreversible binding of FDE to TPM or damage to the TSH receptor. When [125I]bTSH was incubated with the active FDE in Tris-BSA and the mixture was chromatographed on Sephadex G-100 using the same buffer, [125I]bTSH was shifted from an apparent mol wt of 30,000 and eluted at the void volume. Direct binding of [125I]bTSH in fractions from the new, large molecular peak was nil. Addition of a large excess of unlabeled bTSH during preincubation prevented the shift in the elution pattern of [125I] bTSH produced by these FDE.(ABSTRACT TRUNCATED AT 400 WORDS)