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K Brand

Publications and source records attributed to K Brand.

At least 109 records · Page 6Linked to original sources
PubMed1984

Metabolism of 2-oxo-acid analogues of leucine and valine in isolated rat hepatocytes.

Isolated hepatocytes were incubated with 1-14C-labelled 2-oxo-acid analogues of leucine and valine. Decarboxylation and transamination rates were determined by measuring 14CO2 release and the appearance of the corresponding 14C-labelled amino acid. Decarboxylation exceeded transamination with both substrates, the ratio of decarboxylation/transamination being 3.4 for 4-methyl-2- oxovalerate and 78 for 3-methyl-2-oxobutyrate. Urea synthesis and ammonia utilization were not significantly decreased by the 2-oxo-acids even under conditions of stimulated urea production. To see if dietary treatment can alter the decarboxylation/ transamination ratio in favour of transamination rats were fed diets low in nitrogen content. These diets caused a reduction of the branched-chain 2-oxo-acid dehydrogenase activity by 80% but did not alter the branched-chain amino-acid aminotransferase activity in liver cells. This change in enzyme activities resulted in a marked decrease of the uptake and decarboxylation rates of 4-methyl-2- oxovalerate but did not enhance the conversion to the corresponding amino acid. Reducing the uptake of branched-chain 2-oxo acids by liver might increase the amount of orally administered 2-oxo acids reaching the muscle for transamination thus improving the nitrogen-sparing effect of these compounds.

Ammonia↗
PubMed1984

Comparative studies between rates of incorporation of branched-chain amino acids and their alpha-ketoanalogues into rat tissue proteins under different dietary conditions.

Male albino rats (110-120 g) were fed for 10 days on an amino acid diet low in nitrogen (nitrogen = 1.05%) devoid of valine, leucine and isoleucine and supplemented with branched-chain alpha-ketoacids (9.4%) (BCKA-diet). Pair-fed controls received an isocaloric diet (AA-diet) which contained the three branched-chain amino acids (1.4%) instead of the alpha-ketoacids (nitrogen = 1.2%). A third group was fed on a standard diet. Measuring rates of incorporation of radioactive leucine, valine and their corresponding alpha-ketoacids into liver, kidney, heart and brain proteins of rats fed on a standard diet revealed that in liver, branched-chain alpha-ketoacids are incorporated to a lesser extent than the corresponding amino acids. The same was also observed with the BCKA-diet, while the AA-diet reduced BCAA incorporation with the consecutive improvement of the relative incorporation of BCKA over that of BCAA. Injection of branched-chain amino- and alpha-ketoacids results in equal rates of incorporation in kidney and heart proteins. Injecting branched-chain alpha-ketoacids leads to higher incorporation rates in brain than injecting branched-chain amino acids. Thus rates of incorporation of branched-chain alpha-ketoacids differ dependent on the tissue and on the diet applied. They are not consistent with those of branched-chain amino acids.

Amino Acids, Branched-Chain↗
PubMed1983

A dipeptidyl carboxypeptidase in brain synaptic membranes active in the metabolism of enkephalin containing peptides.

A dipeptidyl carboxypeptidase activity has been localized in synaptic plasma membranes which have been prepared from isolated rat brain cortical synaptosomes. The specificity of this proteolytic activity towards various synthetic and biological active peptides is compared to the peptidase activities of intact synaptosomes. In contrast to the synaptosomal peptidases which are capable of cleaving all peptide bonds of Met-enkephalin-Arg6-Phe7 the peptidase activity associated with the synaptic plasma membrane exclusively hydrolyses a dipeptide from the carboxyl terminus of all hepta- and hexapeptides tested. The fact that this dipeptidyl carboxypeptidase does not cleave the Gly3-Phe4 peptide bond of Met-enkephalin suggests that this enzyme is different from "enkephalinase". The synaptic membrane dipeptidyl carboxypeptidase is inhibited by metal chelating agents and thiols but is not affected by compounds known to inhibit serine proteases, thermolysin and "enkephalinase".

Amino Acid Sequence↗
PubMed1983

Processing and degradation of met-enkephalin by peptidases associated with rat brain cortical synaptosomes.

The generation of met-enkephalin (Tyr1-Gly2-Gly3-Phe4-Met5) from met-enkephalin-Arg6-Phe7 and subsequent degradation of the liberated peptides to the free amino acids by rat brain cortical synaptosomes in vitro was demonstrated by HPLC and amino acid analyses. Kinetic measurements of the individual steps of met-enkephalin processing and degradation upon incubation with synaptosomes revealed the following sequence of cleavage: 1. Hydrolysis of the Met5-Arg6 peptide bond, generating met-enkephalin and the dipeptide Arg-Phe. Captopril and EDTA inhibit this reaction. 2. Hydrolysis of the Tyr1-Gly2 peptide bond, generating Tyr and a tetrapeptide. Puromycin (ID50 = 5 X 10(-5) M) and parahydroxymercuribenzoate (ID50 = 5 X 10(-4) M) inhibit this reaction. 3. Hydrolysis of the Gly3-Phe4 peptide bond. Parahydroxymercuribenzoate (ID50 = 5 X 10(-4) M) inhibits this reaction completely. 1 mmol liter-1 Puromycin does not inhibit this reaction. 4. Hydrolysis of the Phe4-Met5 peptide bond. 5. Hydrolysis of the Gly2-Gly3 peptide bond. The pH optimum of all cleavage reactions was found to be around 7.8.

Amino Acids↗
PubMed1983

Effect of benzoate on glycine metabolism in rats fed branched-chain alpha-ketoacids.

Male albino rats (110-120 g) were fed for 9 days an amino acid diet low in nitrogen (nitrogen, 1.05%) devoid of valine, leucine and isoleucine and supplemented with branched-chain alpha-ketoacids (9.4%) (BCKA diet). Pair-fed controls received an isocaloric diet (AA diet), which contained the three branched-chain amino acids (1.4%) instead of the alpha-ketoacids (nitrogen, 1.2%). A third group was fed a standard diet. The animals were divided into three groups of 12 animals, and 6 animals of each group were injected with benzoate (210 mumol/100 g) for 9 days. On day 10 of the experiment the activity of the glycine cleavage system and glycine concentrations were measured in liver; glycine, serine, ammonia, urea, glutamine and glutamate concentrations were determined in plasma on day 10 and determinations of urinary urea, ammonia, glutamine, glutamate, hippuric acid and benzoate were carried out on days 0 and 10 of the experiment. Feeding BCKA led to an increase of plasma and hepatic glycine levels. Activity of hepatic glycine synthesis was reduced in the BCKA-fed group, whereas compared to the AA-fed group no increase in the decarboxylation activity was observed. Injection of benzoate led to an increase in hepatic glycine decarboxylation and glycine synthesis activity in the BCKA-fed group. Plasma glycine levels did not fall, and there was no change in any of the metabolites measured in urine and plasma.

Animals↗
PubMed1983

Some properties of aminopeptidase associated with rat brain cortical synaptosomes.

To understand the breakdown of peptides in the brain, we studied the aminopeptidase associated with synaptosome particles. A continuous spectrophotometric assay in stirred cuvettes was used to follow the kinetics of inactivation by EDTA and by diethyl pyrocarbonate. The sensitivity of the enzyme towards puromycin and leucine hydroxamate was also determined. The results are consistent with the presence of a single species of aminopeptidase in freshly prepared synaptosome. This enzyme is capable of degrading Met-enkephalin in vitro and is distinct from microsomal leucine aminopeptidase. Storage of synaptosomes by freezing and subsequent thawing changed some properties of the enzyme and partially solubilized the enzyme. These studies suggest that there are advantages in studying the enzyme in its native particle-bound state.

Aminopeptidases↗
PubMed1982

Inhibition of leucine aminopeptidase by amino acid hydroxamates.

Amino acid hydroxamates are strong competitive inhibitors of leucine aminopeptidase from porcine kidney. The side chain specificity for inhibition correlates well with substrate specificity. L-Leucine hydroxamate (Ki = 14 microM) protects the enzyme from inactivation by EDTA and is presumed to be a bidentate ligand of the zinc at the active site. A substituted beta-mercaptoketone which may bind in a similar way is also a potent inhibitor (Ki = approximately 1 microM). The binding of these inhibitors suggests a mechanism for this enzyme in which a zinc-bound hydroxide ion participates in concerted proton-transfer processes, while the coordination and charge field at the zinc atom remain unchanged.

Amino Acids↗
PubMed1982

Carbon balance studies of glucose metabolism in rat cerebral cortical synaptosomes.

Synaptosomes were isolated from rat cerebral cortex and incubated with [U-14C]-, [1-14C]- or [6-14C]glucose. Glucose utilization and the metabolic partitioning of glucose carbon in products were determined by isotopic methods. From the data obtained a carbon balance was constructed, showing lactate to be the main product of glucose metabolism, followed by CO2, amino acids and pyruvate. Measuring the release of 14CO2 from glucose labelled in three different positions allowed the construction of a flow diagram of glucose carbon atoms in synaptosomes, which provides information about the contribution of the various pathways of glucose metabolism. Some 2% of glucose utilized was calculated to be degraded via the pentose phosphate pathway. Addition of chlorpromazine, imipramine or haloperidol at concentrations of 10(-5) M reduced glucose utilisation by 30% without changing the distribution pattern of radioactivity in the various products.

Amino Acids↗
PubMed1981

Metabolism of 2-oxoacid analogues of leucine, valine and phenylalanine by heart muscle, brain and kidney of the rat.

[1-14C]-Labelled 2-oxoacid analogues of leucine, valine and phenylalanine were used to study the metabolism of these 2-oxoacids in the brain, kidney and heart muscle of rats. By following the 14CO2 release during 30-60 min of incubation at 37 degrees C the decarboxylation rate was determined and measurement of the 14C-incorporation into the corresponding amino acid yielded the transamination rate. From these rates, decarboxylation/transamination ratios could be calculated which are indicative for the metabolic fate of the 2-oxoacid in the various organs. The results obtained show that all three tissues are capable of utilizing the 2-oxoacid analogues of leucine, valine and phenylalanine, however, to a different extent: kidney greater than heart muscle greater than brain. The decarboxylation/transamination ratios reveal that the branched-chain 2-oxoacids are predominantly decarboxylated in kidney and heart muscle while in brain they are mainly transaminated. The ratios calculated for phenylpyruvate in all tissues are within 0.19 and 0.36, indicating that this 2-oxoacid is preferentially transaminated. The results are discussed with respect to possible dietary alterations of enzymes involved in 2-oxoacid metabolism in order to improve transamination of these compounds.

Aminooxyacetic Acid↗
PubMed1981

Influence of dietary nitrogen intake on mammalian branched chain alpha-keto acid dehydrogenase activity.

Male albino rats (100-130 g) were fed diets of varying casein contents for a period of six days. Increasing the casein content from 0.38% to 22% led to an elevation of hepatic branched chain alpha-keto acid dehydrogenase (BCKAD) activity. Km values measured with alpha-ketoisocaproate and alpha-ketoisovalerate as substrates were not influenced by the dietary treatment. Replacing the casein content (22%) of the diet by an equivalent amount of amino acids resulted in a significant decrease in hepatic BCKAD activity. The decrease in Vmax values was not accompanied by changes in Km-values. No enhancement of hepatic enzyme activity was observed when increasing the amino acid content of the diet from 17.5% to 25%. Changes in enzyme activity due to alterations of the dietary nitrogen source were not observed in kidney and brain.

3-Methyl-2-Oxobutanoate Dehydrogenase (Lipoamide)↗
PubMed1980

Effect of starvation and refeeding a high-protein or high-carbohydrate diet on lipid composition and glycogen content of rat livers in relation to age.

Rats of two different ages (72 and 490 days) were subjected to a 3-day fast and then refed for the same period a high-protein (89%) or high-carbohydrate (99%) diet. This experimental model served to study whether alterations occur in the content of the main lipid components, triacylglycerol, phospholipids and cholesterol in liver. It also examined whether the aging process itself has any effect on the lipid pattern. With the high-protein diet the content of all lipids returned to or nearly to the control level regardless of age. Refeeding the 72-day-old rats the high-carbohydrate diet led to a 2.4-fold overshoot in the triacylglycerol content, while the phospholipids and the cholesterol remained far below their control levels. In the 490-day-old rats the total lipids, phospholipids and cholesterol remained at the starvation level. With aging, the total cholesterol content showed a marked increase of about four-fold when corrected for growth.

Aging↗