Search PubMed⌕ Search

Biomedical subjects

B Borrebaek

Publications and source records attributed to B Borrebaek.

At least 37 records · Page 2Linked to original sources
PubMed1979

Arsenite inhibits beta-oxidation in isolated rat liver mitochondria.

A partial inhibition of acylcarnitine oxidation by arsenite in rat liver mitochondria has been studied. This inhibition is confined to the thiolase(s). The inhibition was observed also in the presence of malate, indicating no selective effect on ketogenesis. Ketogenesis from acetyl-CoA was inhibited by arsenite. Mitochondrial CoA was acylated by acylcarnitine nearly as rapidly in the presence of arsenite as in its absence. Thus, arsenite did not interfere with the availibility of CoA in the mitochondria. No effect of arsenite on enzymes of beta-oxidation other than the thiolase(s) was observed. When arsenite and acylcarnitine were added simultaneously to mitochondria, there was a delay before maximal inhibition of oxygen uptake occurred. When the mitochondria were preincubated with arsenite before addition of acylcarnitine, the inhibitory effect on oxygen utilization was initially large, but then partially repealed. Similar time delays were observed in the activity of acetoacetyl-CoA thiolase of disrupted mitochondria depending on the sequence of arsenite and acetoacetyl-CoA addition. It is suggested that substrate and arsenite complete for the reactive sulfhydryl group at the active site of the thiolase(s).

Acetyl-CoA C-Acetyltransferase↗
PubMed1978

The effects of clofibrate feeding on the metabolism of palmitate and erucate in isolated hepatocytes.

The metabolism of palmitate and erucate has been investigated in hepatocytes isolated from control rats and from rats fed 0.3% clofibrate. Clofibrate increased the oxidation of [1-14C]palmitate 1.5 to 2-fold while the esterification was decreased. At a high concentration of palmitate (1.5 mM), the total rate of fatty acid metabolism was stimulated. Clofibrate stimulated both the oxidation (3.5 to 5-fold) and the esterfication (1.7-fold) of [14-14C]erucate. Erucate undergoes chain-shortening in isolated liver cells. This chain-shortening was stimulated at least 2-fold by clofibrate feedings. The isolated mitochondrial fraction from clofibrate-fed rats showed an increased capacity for oxidation of short-chain acylcarnitines (including acetylcarnitine), while the oxidation of palmitoyl- and erucoylcarnitine showed little change. It is suggested that erucate is shortened by the recently detected beta-oxidation system of peroxisomes.

Acyltransferases↗
PubMed1976

The glycerophosphateacyltransferases and their function in the metabolism of fatty acids.

1. From different studies on the cellular localization, postional specificity, and regulatory properties of acyl-CoA: glycerophosphate acyltransferase (EC 2,3,1.15) AND ACYL-CoA: 1-ACYLGLYCEROPHOSPHATE ACYLTRANSFERASE (EC 2,3,1....) the following conclusions can be drawn: The glycerophosphate acyltransferase is localized in the endoplasmatic reticulum (microsomes) and in the outer membrane of the mitochondria of the animal cell. Its reaction product is 1-acylglycerophosphate (1-lysophosphatidic acid). The mitochondrial enzyme shows a high preference for saturated fatty acids while the microsomal enzyme is less specific (alternatively the microsomes contain more than one glycerophsophate acyltransferase). 2. The 1-acylglycerphosphate acyltransferase is localized in the endoplasmatic reticulum (microsomes) in the animal cell. Possibly a minor fraction of this enzyme is localized to the outer membrane of the mitochondria. This enzyme shows a strong preference for unsaturated fatty acids. 3. Both the microsomal and the mitochondrial dihydroxyacetonephosphate acyltransferase show similar fatty acid specificity as the corresponding glycerophosphate acyltransferases. It cannot be excluded that dihydroxy-acetonephosphate and glycerophosphate are acylated by the same enzymes. 4. The activity of the glycerophosphate acyltransferase(s) in the liver decreases in fasting or fat feeding and increases upon feeding of carbohydrate. The activity of carnitine palmityltransferase varies exacty opposit. These enzymes do not show dietary variations in heart and adipose tissue. 5. Under the otherwise identical conditions the rate of carnitine acylation in isolated mitochondria decreases more than the rate of glycerophosphate acylation when the concentration of palmityl-CoA is reduced. 6. In isolated liver cells (which has lost most of their carnitine) addition of carnitine increases the rate of fatty acid oxidation and decreases the rate of triglyceride formation. 7. Glycerol and fructose lower the rate of fatty acid oxidation, probably by lowering the levels of acyl-CoA and acyl-carnitine in the cells. 8. It is concluded that the relative activities of glycerophosphate acyltranse and carnitine palmityltransferase probably influence the fate of fatty acids in the cell.

Acyltransferases↗
PubMed1976

The role of acyltransferases in fatty acid utilization.

Glycerophosphate and carnitine as competing acyl acceptors and the oxidation of different long chain acylcarnitines in isolated mitochondria have been investigated. In the presence of low concentrations of palmitate, glycerophosphate is the preferred acyl acceptor in isolated liver mitochondria, indicating that the glycerophosphate acyltransferase has a lower Michaelis constant (Km) for palmityl-CoA than has the carnitine palmityltransferase. Liver mitochondria from starved rats make more palmitylcarnitine and less palmitylglycerophosphate than do mitochondria from starved/refed rats. In heart mitochondria glycerophosphate has less effect on palmitylcarnitine formation because of glycerophosphate acyltransferase activity is lower and the carnitine palmityltransferase activity is higher in liver mitochondria. Carnitine esters of C22 fatty acids, especially erucic acid (22:1omega9,cis) are more slowly oxidized compared to palmitylcarnitine in heart than in liver mitochondria. They also inhibit palmitylcarnitine oxidation. The inhibition is relatively stronger in heart than in liver mitochondria. These observations are discussed in relation to organ differences in the utilization of fatty acids.

Acylation↗
PubMed1975

Acylation of carnitine and glycerophosphate in suspensions of rat liver mitochondria at varying levels of palmitate and coenzyme A.

Rates of acylation of carnitine and glycerophosphate in suspensions of isolated rat liver mitochondria were measured at varying levels of palmitate and coenzyme A in the presence of ATP. Addition of glycerophosphate caused considerable reduction in carnitine acylation at low, but not at high palmitate levels. Glycerophosphate acylation was less reduced by added carnitine. These results can be explained by a lower Km(acyl-CoA) in glycerophosphate acylation than in carnitine acylation. High levels of free coenzyme A caused inhibition of carnitine acylation, while the inhibitory effect of glycerophosphate acylation was small. Competition between palmitate and acyl-CoA for binding sites on protein (e.g., albumine added to the mitochondrial suspensions) was indicated by stimulation of carnitine acylation by palmitate with palmityl-CoA added as substrate in the absence of ATP. Refeeding a carbohydrate-rich diet to previously fasted rats resulted in increased glycerophosphate acylation and decreased carnitine acylation in isolated liver mitochondria.

Acylation↗