Dihydrogen activation, inhibition

As indicated in Fig. 1, nitrogenase can reduce substrates other than Na. In the absence of other reducible substrates it will reduce protons to dihydrogen, but it can also reduce a number of other small triple-bonded substrates, as indicated in Section V,E,1. Large substrates are not reduced efficiently, indicating physical limitations on access to the enzyme s active site. CO is a potent inhibitor of all nitrogenase substrate reductions except that of the proton to Ha. In the presence of CO the rate of electron transfer is generally not inhibited, but all electrons go toward the production of Ha. 

In addition to bromide and iodide, V-BrPO can catalyze the oxidation of chloride [64]. As mentioned previously and discussed more fully later, a distinct enzyme, vanadium chloroperoxidase, has also been discovered. Originally it was thought that V-BrPO could only catalyze the oxidation of bromide and iodide by dihydrogen peroxide. In fact, under the standard mcd bromoperoxidase assay conditions, in which the V-BrPO concentration is ca. nanomolar, very little, if any, chlorination of mcd is observed. However, it seemed very unusual that V-BrPO could be inhibited by fluoride and bromide, but apparently not by chloride [27], In reinvestigating the halide specificity of V-BrPO, it was discovered that when the enzyme concentration is increased 100-fold to 0.1 pM, chlorination is observed at an appreciable rate [64], The specific chloroperoxidase activity is 0.76 U/mg (under conditions of 1 M certified 100% bromide-free KC1, 2 mMH202, 50 pM... 

Hydrogenation of the double bond in position 9-10 of the lysergic acid moiety results in fundamental changes in the pharmacodynamic action, as a comparison of the activity spectra of ergotamine (Fig. 7a) and dihydroergotamine (Fig. 7c) makes evident. The vasoconstrictor and uterotonic effects of the dihydrogenated derivative have been markedly attenuated, as has the stimulation of central structures, so that these effects are hardly elicited by therapeutic doses. On the other hand the adrenolytic effect and the central inhibition of the vasomotor centers are markedly enhanced. This is manifested clinically in vasodilatation, hypotension, and a certain sedative action. 

Choline bromide succinate, 998 Choline chloride, 466 Choline chloride carbamate, 427 Choline chloride succinate, 998 Choline citrate, 466 Choline dihydrogen citrate, 466 Choline salicylate, 965 Choline theophyllinate, 1011 Cholinesterase activity, assay kit, 1171 quantification in serum, 23 test for inhibition of, 6 Chondodendron tomentosum, 1057 Chromatographic performance, evaluation of, 189 Chromatography, adsorption, 204... 

PNG was determined under reaction conditions 7. The reaction comprised two steps in one reaction tube. The first step was conversion of PNG into PN at 30 °C for 2 h by p-glucosidase in reaction mixture 7, which contained a high concentration of sodium dihydrogen phosphate to inhibit phosphatase activity in the almond p-glucosidase preparation. After the two-hour reaction, NaOH and Tris-HCl buffer were added to the reaction mixture to make the pH 9.0. Then, PLDH, NAD" ", PNOX and FAD were added to the reaction mixture, which was additionally incubated at 30 °C for 1 h. 4-PLA in this reaction mixture is derived from PNG, PN and PL. Subtraction of the amount of 4-PLA derived from PN and PL gives the amount of 4-PLA derived from PNG. 

Works on the inhibition of dihydrogen combustion by additives of various substances play an important role in studies of this reaction. Among active species formed in this system atomic hydrogen is formed in the greatest amount and provides chain branching (see Table 12.1). Atomic hydrogen rapidly enters into such reactions as H abstraction... 

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