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J Fejes

Publications and source records attributed to J Fejes.

6 recordsLinked to original sources
PubMed1979

Mechanism-controlled stereospecificity. Acylation of subtilisin with enantiomeric alkyl and nitrophenyl ester substrates.

The activation parameters of acylation of subtilisin with alkyl and p-nitrophenyl esters of N-acylamino acid enantiomers were determined. It was found that (1) the activation entropy is much higher with the nitrophenyl esters than with the corresponding methyl esters, (2) the difference in rate constants between enantiomers is 10(4)--10(5) with methyl esters whereas it is only of the order of 10 with nitrophenyl esters. The results indicate that the catalytic mechanism is simpler for nitrophenyl esters than for alkyl esters. The simple mechanism requires only general base catalysis, and thus permits more freedom of motion in the transition state, whereas the complex mechanism involves both general base and general acid catalysis. Furthermore, the strikingly low enantiomeric specificity with nitrophenyl esters indicates that not only binding but also the catalytic mechanism is an important factor in determining the stereospecificity of an enzyme. The activation parameters for enantiomeric nitrophenyl ester reactions suggest that structurally related substrates can be transformed by the enzyme in different conformations which may be energetically similar or not. The energetically different conformations may account for the activation enthalpy-entropy compensation.

Acylation↗
PubMed1979

Endogenous urea secretion into the sheep gastrointestinal tract.

Urea turnover and the proportion of endogenous urea secreted and excreted in the saliva, the bile, the pancreatic juice and the urine and directly across the wall of the digestive tract was studied in 6 experiments, after a single i.v. dose of labelled 15N, in two adult sheep weighing 49 and 50 kg, with permanent biliary and pancreatic fistulus and with an exteriorized right parotid duct. It was found that, of the total amount of endogenous urea secreted into the animals' digestive tract (0.2694+/-0.0138 mg/min/kg b.w.), 10.27+/-0.94% reached the contents in the saliva, 2.12+/-0.28% in the bile and 0.66+/-0.08% in the pancreatic juice, and that 86.95+/-2.1% was secreted into the gastrointestinal tract, across its wall, from the blood capillaries. Exogenous turnover amounted to 0.3228+/-0.192 mg/min/kg. Of the total amount of 476.6 mg i.v. injected 15N urea, 274.1+/-8.86 mg was excreted in the urine 5.1+/-0.9 mg in the bile, 3.19+/-0.06 mg in the pancreatic juice, 4.96+/-0.76 mg via the right parotid gland and 9.34+/-1.09 mg in the faeces. The results show that the quantitatively most important part of the recirculation of endogenous urea is its passage from the blood across the wall of the gastrointestinal tract into its contents.

Animals↗
PubMed1978

[Passage of endogenous urea into a jejunum of sheep].

The quantitative passage of urea-N and ammonia N into an isolated jejunum of the merino sheep was studied within one hour before, and 1, 2, 3, 4 and 5 hours after feeding. The total amount of ammonia-N that passes to the isolated jejunum [to both proximal and distal part] within an hour is small [0.28-0.35 mg h-1], without any significant differences in passage prior to and after feeding. However, urea-N secretion is high, especially secretion to the fore part of the jejunum [ 3.35-3.62 mg h-1]. The passage of urea-N to the rear part of the jejunum is about 1/3 lower than passage to the fore part. Urea solution of higher concentration than in peripheral blood was incubated in the isolated jejunum to study an adverse phenomenon: absorption of urea from intestinal lumen to the blood. The levels of urea in peripheralblood and in incubate reached a balance state within one hour after administration. The data obtained testify to the permeability of the wall of the gut for urea in both directions.

Ammonia↗
PubMed1977

Utilization of 15N-urea administered into the sheep small intestine.

The retention and excretion of intrajejunally administered 15N-urea was studied in four experiments on two sheep with a permanently fistulated small intestine. In the first 7 days after the administration of 2 g 15N-urea, 18.26% was excreted in the faeces and 19% in the urine; 62.74% was retained in the organism. Urinary excretion took place mainly on the first day and from the 3rd to the 7th day no 15N was present in the urine. The rate of 15N excretion in the faeces was roughly the same for the first 4 days and then fell; on the 7th day there was no 15N in the faeces. The proportion of 15N-urea retained in the organism and excreted in the urine was 81% showing that urea in the ruminant gastrointestinal tract is largely linked up into metabolic circulation as part of the general exchange of nitrogenous compounds.

Animals↗
PubMedDate not supplied

[Participation of bile and pancreatic juice in the secretion of endogenous nitrogen into the digestive tract of sheep].

The amount of bile and pancreas juice excreted and the amount of urea-N and total-N discharged by the bile and pancreas juice into the duodenum was established in eight experiments with two full-grown sheep of the merino species and with a live weight of 50 kg. For this purpose they had permanent cannulae for the measuring of the passage of bile and pancreas juice which was carried out at 1-hour intervals in the course of 24 hours beginning at 7 o'clock in the morning. During 24 hours 245.3 +/- 6.3 mg urea and 4,418.1 +/- 85.8 mg total-N were discharged with bile and pancreas juice, of which 186.9 +/- 9.6 mg urea and 2,095.3 +/- 119.7 mg total-N were discharged in the bile and 58.5 mg urea-N and 2322.8 +/- 52.9 mg total-N were discharged in the pancreas juice. The amount of pancreas juice secreted and its nitrogen is constant in the course of the day (between 7 a.m. and 7 p.m.) and of the night (between 7 p.m. and 7 a.m.) In the daytime the secretion of bile and its nitrogen is significantly higher than at night. (p less 0.05). The concentration of urea-N and of total-N remained constant throughout the whole day. The amount of bile and pancreas juice secreted correlated greatly significantly with the amount of nitrogen (urea-N and total-N) discharged in bile and pancreas juice. The results indicate that, within the enterohepatalous circulation of endogenous urea, its secretion in bile and pancreas juice is quantitatively less important. The amount of total-N which gets from the endogenous pool into the duodenum is high and corresponds to approximately one third of the metabolic nitrogen which the animals consume daily with their feed.

Animals↗