Role of steric interactions in the delta opioid receptor selectivity of [D-Pen2, D-Pen5]enkephalin

In order to assess the individual effects of each of the 3-methyl groups in residue 2 of [D-Pen2, D-Pen5]enkephalin on binding affinity to mu and delta opioid receptors, (2S,3S)methylcysteine ((3S)Me-D-Cys) and (2S,3R)methylcysteine ((3R)Me-D-Cys) were synthesized and incorporated into the analogs, [(3S)Me-D-Cys2, D-Pen5] enkephalin and [(3R)Me-D-Cys2, D-Pen5]enkephalin. Of these analogs, [(3S)Me-D-Cys2, D-Pen5]enkephalin appears from 1H n.m.r. spectra to assume a conformation similar to those of [D-Pen2, D-Pen5]enkephalin and the less delta receptor-selective, but more potent, [D-Cys2, D-Pen5]enkephalin. Assessment of binding affinity to mu and delta receptors revealed that [(3S)Me-D-Cys2, D-Pen5]enkephalin exhibits delta receptor affinity intermediate between [D-Pen2, D-Pen5]enkephalin and [D-Cys2, D-Pen5]enkephalin while its mu receptor affinity is similar to that of [D-Cys2, D-Pen5]enkephalin. These results suggest that, for [D-Pen2, D-Pen5]enkephalin, adverse steric interactions between the D-Pen2 pro-R methyl group and the mu receptor binding site lead to the low mu receptor binding affinity observed for this analog. By contrast, both the pro-R and pro-S D-Pen2 methyl groups lead to minor steric interactions which contribute to the somewhat lower delta receptor affinity of this compound.

Stereochemical course of the reaction catalyzed by the pyridoxal phosphate-dependent enzyme 1-aminocyclopropane-1-carboxylate synthase

(+/-)-S-adenosyl-DL-(3R*,4S*)-[3,4-2H2]-methionine [a 1:1 mixture of (3R,4S) and (3S,4R)] and (+/-)-S-adenosyl-DL-(3R*,4R*)-[3,4-2H2]methionine [a 1:1 mixture of (3R,4R) and (3S,4S)] were synthesized from (Z)- and (E)-[1,2-2H2]ethene, respectively. Key steps in the synthesis were the antiperiplanar addition of methanesulfenyl chloride to (Z)-[1,2-2H2]ethene, to give a 1:1 mixture of (R,R)- and (S,S)-1-chloro-2-(methylthio)[1,2-2H2)ethane, followed by alkylation with sodium acetamidomalonate and hydrolysis to give an equal mixture of four stereoisomers of [3,4-2H2]methionine [(2R,3R,4S), (2R,3S,4R), (2S,3R,4S), and (2S,3S,4R)]. The other four stereoisomers of [3,4-2H2]methionine were prepared from (E)-(1,2-2H2]ethene. The two sets of stereoisomers of [3,4-2H2]methionine were chemically converted to S-adenosylhomocysteine, methylated to give the corresponding (+/-)-S-adenosyl-DL-methionines, and then incubated with 1-aminocyclopropane-1-carboxylate synthase partially purified from tomato (Lycopersicon esculentum, L.) pericarp tissue. The stereochemistry of the resulting samples of 1-aminocyclopropane-1-carboxylic acid was determined by comparison with the 1H NMR of the chemically synthesized and regio- and stereo-specifically deuterated compound. The results indicate that the hydrogens at the beta carbon of the methionine portion of S-adenosylmethionine do not participate in the reaction and that the ring closure occurs with inversion of configuration at the gamma carbon of the methionine portion of S-adenosyl-methionine, probably through a direct SN2-type displacement of the 5'-methylthio-5'-deoxyadenosine moiety by a carbanion equivalent formed at the alpha carbon of the methionine portion of S-adenosylmethionine.

A comparative study on esterases from three species of Raillietina

The multiplicity of soluble esterases in Raillietina tetragona, R. echinobothrida and R. cesticillus was studied by use of slab polyacrylamide gel electrophoresis. Five fractions of esterase activity were observed in R. tetragona, seven in R. echinobothrida and three in R. cesticillus. The various fractions of esterase activity of closely related species of Raillietina showed differential behaviour towards various chemicals. Based on the inhibitory effect of inhibitors p-CMB, EDTA, malathion, silver nitrate and eserine sulphate, the various esterases have been classified into arylesterase, carboxylesterase, acetylesterase and cholinesterase.

Isozyme pattern of lactate and malate dehydrogenases of Gastrothylax crumenifer (Trematoda: Amphistomatidae) from different hosts

Isozyme pattern of lactate dehydrogenase (LDH) and malate dehydrogenase (MDH) of Gastrothylax crumenifer from sheep, goat and buffalo was studied using polyacrylamide gel electrophoresis. LDH of G. crumenifer from buffalo, goat and sheep consists of four fractions, three fractions and two fractions, respectively. The parasite from buffalo shows two fractions of MDH, whereas those from goats or sheep show only a single fraction. The significance of these results is discussed.

Activation of prophenol oxidase in the liver fluke Fasciola gigantica Cobbold

Activation of prophenol oxidase of Fasciola gigantica has been demonstrated using biochemical and electrophoretic techniques. Based on the difference in the electrophoretic pattern of the proenzyme and activated enzyme, a probable mechanism of activation of the prophenol oxidase is suggested. A role for Mehlis' gland in the activation process is suggested. The significance of the results is discussed in the light of related studies in insects.

Prophenoloxidase and its activation in Paraplerurus sauridae (Digenea: Hemiuridae)

Electrophoretic studies on the phenol oxidase of Paraplerurus sauridae indicated that the enzyme exists in multiple forms. Based upon the difference in the electrophoretic pattern before and after activation of the enzyme, the possible mechanism of activation of the enzyme is suggested. The role of Mehlis' gland in the activation of the enzyme is also discussed. The various enzyme fractions showed differences in ability to utilise mono- and diphenolic substrates. The significance of the differences are discussed.

Specificity of the enzyme phenoloxidase and possible metabolic pathway of sclerotization in Paraplerurus sauridae

Activity of phenoloxidase in Paraplerurus sauridae on various substrates has been studied spectrophotometrically. The enzyme was shown to be nonspecific and capable of oxidizing both mono- and diphenols. It showed greater activity to some of the mono- and diphenols. Substrates with acidic side chain were oxidized less effectively. Based on the activity of the enzyme on different substrates, a possible metabolic pathway of sclerotization has been suggested.