Kinetics of the reaction of nitrous acid with model compounds and proteins, and the conformational state of N-terminal groups in the chymotrypsin family

The kinetics of the reaction of nitrous acid at 4° and pH 4.0 with various amino acids, peptides, and proteins were studied. The reaction with isoleucine methyl ester was found to have a linear dependence on the square of the HONO concentration showing that N2O3 was the reactive species. Third order nitrosation rate constants of primary amino groups showed a correlation with their pK values. They were calculated for the concentration of the unprotonated species to give intrinsic reactivities. The rate of nitrosation of acetyltryptophan to give N-nitrosoacetyltryptophan was found to be a linear function of the nitrous acid concentration. This nitrosation therefore follows a different mechanism. The reaction of nitrous acid with tyrosine residues was examined by spectrophotometry. The reaction was negligible compared to that of other groups. Acetylhistidine and imidazole did not react. Reactivities for α-amino groups, ε-amino groups, and other residues in proteins were compared. The conformational state of the N-terminal residues in serine proteinases, as revealed from their reactivities, is discussed in detail. It is concluded that nitrous acid reacts preferentially with "surface" residues and is a useful tool for exploring conformational states of reactive groups in proteins, especially α-amino groups and indole rings.

Ascorbate-nitrite reaction: possible means of blocking the formation of carcinogenic N-nitroso compounds

The formation of carcinogenic N-nitroso compounds by the chemical reaction between nitrous acid and oxytetracycline, morpholine, piperazine, N-methylaniline, methylurea, and (in some experiments) dimethylamine was blocked by ascorbic acid. The extent of blocking depended on the compound nitrosated and on the experimental conditions. Urea and ammonium sulfamate were less effective as blocking agents. The possibility of in vivo formation of carcinogenic N-nitroso compounds from drugs could be lessened by the combination of such drugs with the ascorbic acid.

Pyridine nucleotide depletion in pancreatic islets associated with streptozotocin-induced diabetes

An investigation of NAD-NADH concentrations in the pancreatic islets of rats following diabetogenic doses (65 mg./kg.) of streptozotocin (SZ), given singly or in combination with nicotinamide or nicotinic acid, is reported. After a given treatment of the animals, the pancreases were surgically removed. The concentrations of NAD and NADH were determined on islets isolated from freeze-dried sections of the pancreas. In the pancreatic islets of untreated control animals, the mean ± S.E. concentrations of NAD and NADU were 1,122 ± 5 7 and 129 ± 28 mμnmoles/gm. dry tissue, respectively. The NAD content of the islets decreased to 27 per cent of the control value one hour and to 12 per cent five hours after administration of SZ. NADH concentration five hours after SZ was significantly reduced from the control value. When nicotinamide was given thirty minutes before SZ, the depletion of NAD associated with SZ administration was prevented. When nicotinamide was given one hour after SZ, the depressed NAD concentration observed at this time was subsequently raised. In contrast, nicotinic acid given thirty minutes prior to SZ did not prevent depletion of NAD. The data suggest that streptozotocin-induced betacell deterioration may be etiologically related to depletion of cellular NAD.

Comparison of liver glycogen synthetase between streptozotocin-diabetic and control rats

Chronic diabetic rat, induced by streptozotocin, showed high blood glucose but low liver glycogen levels. The diabetic liver had subnormal amounts of glycogen synthetase and its I form when assayed in both 8000 × g supernatant and glycogen pellet suspensions. The pellet suspension of the control liver retained 68% synthetase I activity; the diabetic liver retained only 19.5%. The addition of deproteinized liver extract to the pellet suspension increased the activity of synthetase either in the presence or absence of glucose-6-PO4 for both groups of animals to different degrees. The deproteinized liver extract of the diabetic rat contained little ATP, less glucose-6-PO4, and slightly more inorganic phosphate than that of the control rat. The significance of these observations is discussed.