Evaluation of 11-[3H]-tetrodotoxin use in a heterologous receptor binding assay for PSP toxins.

This report describes the preparative scale production of 11-[3H]-tetrodotoxin (TTX) and its evaluation as a substitute for [3H]-saxitoxin (STX) as the radioligand in a receptor binding assay for paralytic shellfish poisoning (PSP) toxins. Restrictions on the world-wide distribution of [3H]-STX imposed by the international Chemical Weapons Convention served as the primary impetus for this study. We have incorporated on a preparative scale, a nonexchangeable tritium label into the TTX molecule at a specific activity of 12.90 Ci/mmol and recovered material of high radiochemical purity (98%). The resulting 11-[3H]-TTX was found to exhibit site-specific binding characteristics in the receptor assay (dissociation constant(K(d))=4.77+/-1.54nM; maximum binding(B(max))=1. 62+/-0.24pmol/mg of synaptosomal protein). The inhibition constant (K(i)) for the assay was 1.46+/-0.28 nM STX equiv. (n=6), with an estimated detection limit of ca. 2-4 ng STX equiv./ml in a sample extract. Moreover, quantitative comparisons indicated that 11-[3H]-TTX could be used interchangeably with [3H]-STX in the receptor assay for determination of PSP toxicity in shellfish and algal extracts without compromising assay performance. We conclude that the 11-[3H]-TTX produced and evaluated herein exhibits physical, chemical and biological characteristics suitable not only for use in the PSP receptor binding assay, but likely for other applications employing [3H]-STX as the radioligand.

Bile acid synthesis. Metabolism of 3 beta-hydroxy-5-cholenoic acid to chenodeoxycholic acid.

Metabolism of 3 beta-hydroxy-5-cholenoic acid to chenodeoxycholic acid has been found to occur in rabbits and humans, species that cannot 7 alpha-hydroxylate lithocholic acid. This novel pathway for chenodeoxycholic acid synthesis from 3 beta-hydroxy-5-cholenoic acid led to a reinvestigation of the pathway for chenodeoxycholic acid from 3 beta-hydroxy-5-cholenoic acid in the hamster. Simultaneous infusion of equimolar [1,2-3H]lithocholic acid and 3 beta-hydroxy-5-[14C]cholenoic acid indicated that the 14C enrichment of chenodeoxycholic acid was much greater than that of lithocholic acid. Thus, in all these species, a novel 7 alpha-hydroxylation pathway exists that prevents the deleterious biologic effects of 3 beta-hydroxy-5-cholenoic acid.

Synthesis of 3 alpha,7 alpha-dihydroxy-5 beta-androstan-17-one.

A short and efficient method for the stereospecific synthesis of 3 alpha,7 alpha-dihydroxy-5 beta-androstan-17-one was accomplished from the readily available 4-androstene-3,17-dione. Key steps are the stereospecific and selective epoxidation of 4,6-androstadiene-3,17-dione, followed by hydrogenations with carefully selected reagents, solvents and reaction conditions.

Bile acid synthesis. Metabolism of 3 beta-hydroxy-5-cholenoic acid in the hamster.

Synthesis of 3 beta-hydroxy-5-[1,2-3H]cholenoic acid has permitted a study of its metabolism in bile-fistula hamsters that received the compound by intravenous infusion. Metabolites in bile were identified by reverse isotope dilution after their complete resolution by high pressure liquid chromatography using muPorasil. Recovery of administered radioactivity ranged from 21-60% in three animals. In each study, lithocholic acid (0.8-4.4%) and chenodeoxycholic acid (7.8-11.3%) were identified as metabolites of 3 beta-hydroxy-5-cholenoate and can be considered primary bile acids in the side-chain pathway of bile acid synthesis beginning with the oxidation of cholesterol to 26-hydroxycholesterol.

Specificity of the cholesterol side-chain cleavage enzyme system of adrenal cortex.

Incubation of lanosta-8, 24-dien-3beta-o1-1,2-3H and lanost-8-en-3beta-o1-1,2-3H with an adrenocortical bovine mitochondrial acetone-dried preparation did not yield any significant (less than 0.01%) 3beta-hydroxy-4, 4, 14-trimethyl-5alpha-pregn-8-en-20-one. Under the same conditions cholesterol-1,2-3H yielded 8.3% pregnenolone. Incubation of (20S)-17alpha, 20-di-hydroxycholesterol-7-3H yielded 0.6 to 1.6% (20S,22R)-17alpha, 20, 22-trihydroxycholesterol, 1.0 to 3.2% of 17alpha-hydroxy-pregnenolone, but no significant (less than 0.02%) (20S,22S)-17alpha,20,22-trihydroxycholesterol. In another experiment incubation of cholesterol-1,2-3H yielded 5% pregnenolone, 0.5% 17alpha-hydroxypregnenolone, 0.2% (20R,22R)-20,22-dihydroxy-cholesterol, but no significant ( less than 0.01%) 17alpha-hydroxy-cholesterol, (20S)-17alpha, 20-dihydroxycholesterol or (20S,22R)-17alpha, 20,22-trihydroxycholesterol.

Exclusion of 20(22)-dehydrocholesterol as an intermediate in the biosynthesis of pregnenolone in bovine adrenocortical mitochondrial acetone-dried powder preparations.

Incubation of (22R)-(22-180)20-hydroxycholesterol with a bovine adrenocortical mitochondrial acetone-dried powder preparation in air yielded (20R, 22R)-20, (22-18O)22-dihydroxy-cholesterol. Incubation of (20S)-(20-18O)22-hydroxycholesterol yielded (20R, 22R)-(20-18O)20,22-dihydroxycholesterol. The formed glycols and the substrates reisolated at the end of the incubations had the same 18O abundance as the starting materials. No significant (20R, 22R)-20,22-dihydroxycholesterol was formed following incubation with either (E)-or (Z)-20, (22)-dehydrocholesterol. (20R,22S)-20, 22-Epoxycholesterol yielded approximately 1/5 of the amount of pregnenolone obtained in a similar incubation with cholesterol. No significant pregnenolone formation was observed with (20R, 22R)-20,22-epoxycholesterol. These results exclude a mechanism for the biosynthesis of (20R, 22R)-20,22-dihydroxycholesterol from the monohydroxylated cholesterol derivatives by way of dehydration followed by epoxidation and hydration. Similarly, the participation of an olefin and an epoxide as intermediates in the transformation of cholesterol to pregnenolone in acetone-dried powder preparations of adrenal cortex mitochondria is unlikely.