Specific labelling of the curare binding sites of acetylcholinesterase and some properties of the modified enzyme

Electric eel acetylcholinesterase was found to selectively react covalently with two molecules of labelled N,N-dimethyl-2-phenylaziridinium chloride (14C-DPA) per unit of 65 000 daltons. One molecule of bound DPA is selectively labilized at pH 9.5. Both alkylated enzyme species (E.2DPA and E.DPA) are inactive toward acetylcholine as substrate, but considerably more reactive toward indophenylacetate (IPA) as substrate. The IPA activities of both the E.2DPA and E.DPA enzymes are unaffected by decamethonium bromide (C10), whereas only the latter suffers inhibition by d-tubocurarine. It is concluded that the enzyme moiety where quaternary effectors interact possesses two topographically distinct classes of sites: one class is essential for C10 inhibition of the esteratic activity, and the other would serve as an allosteric binding site for curare. This latter site may play the role of receptor on excitable membranes.

Some properties of the succinate dehydrogenase of Escherichia coli

Succinate dehydrogenase (SDH) was found in a cell envelope fraction from E. coli. If the enzyme preparation was made in 0.01 M phosphate buffer, pH 7.5, with or without 0.025 M succinate, or in 0.01 M Tris buffer, the activity of the enzyme could be markedly increased by heating the enzyme at 38° for 15 min with 15 mM succinate. Activation did not occur at 0°. If the enzyme preparation was made in Tris buffer containing succinate the enzyme was found to be fully activated. SDH was obtained in a low molecular weight form by extracting an acetone powder of whole cells with buffer. This enzyme had a molecular weight of about 100 000 and could be purified free from cytochrome.

Effect of spironolactone and ethylestrenol on benzo(a)pyrene hydroxylase (EC 1.14.1.1) and other chemical constituents of hepatic microsomes

In rats, spironolactone and ethylestrenol, like phenobarbital, enhance the NADPH-dependent hydroxylation of benzo(a)pyrene in the hepatic microsomal plus supernatant fraction and increase liver weight and microsomal phospholipid content as well as NADPH cytochrome c reductase and diaphorase activities, but only ethylestrenol and phenobarbital influence the microsomal protein content and cytochrome P-450 level.Neither spironolactone, ethylestrenol, nor phenobarbital affects NADH cytochrome c reductase and diaphorase activities, and only phenobarbital alters the cytochrome b5 level.These findings indicate that, while both the steroids and phenobarbital stimulate microsomal mixed-function oxidation, there are qualitative and quantitative differences between their action.