Carnitine palmitoyltransferase activities (1 and 2) and the rate of palmitate oxidation in liver mitochondria from diabetic rats

Prolonged effect of single carnitine administration on fasted carnitine-deficient JVS mice regarding their locomotor activity and energy expenditure

Carnitine is an essential cofactor for the oxidation of fatty acid in the mitochondria and an efficient therapeutics for primary carnitine deficiency. We herein analyzed the prolonged effects of carnitine on the reduced locomotor activity and energy metabolism of fasted carnitine-deficient juvenile visceral steatosis (jvs(-/-)) mice. We found that a single carnitine administration to 24-h fasted jvs(-/-) mice in the morning increased both the locomotor activity and oxygen consumption at night not only on the same day, but also on the next day, when the carnitine levels in the blood and tissues were already as low as at the original carnitine-deficient state. We also found that fat utilization for energy production significantly increased under fasting even in jvs(-/-) mice and was stimulated in the carnitine-administrated fasted jvs(-/-) mice at night, in comparison to that observed in the saline-administered jvs(-/-) mice, at least for 2 days even under the low plasma and tissue carnitine levels. These results suggest that the low tissue carnitine levels are therefore not the sole rate-limiting factor of general fatty acid oxidation in carnitine-deficient jvs(-/-) mice.

Concept of the noncoenzymatic thiamine effect

The experimental and clinical data on different aspects of vitamin and hormone relationships have been summarized in the form of a general concept of the noncoenzymatic thiamine effect, on the basis of a number of premises: (1) discovery of tissue factors limiting the manifestation of the specific activity of administered thiamine (the presence of a tissue buffer depot of easily accessible coenzymes, and lack of free apoenzymes); (2) evidence of a thiamine effect on the pancreatic insulin-synthesizing function; (3) stimulation of metabolic thiamine effects, including the effects of insulin administration on thiamine-dependent enzymes; (4) determination of the features of hormonal control of thiamine metabolism in the body; (5) confirmation of the predictive force of the concept by clinical trials of the new strategy of thiamine therapy.

Dietary fatty acids influence the activity and metabolic control of mitochondrial carnitine palmitoyltransferase I in rat heart and skeletal muscle

The fatty acid composition of the diet has been found to influence the activity and sensitivity of mitochondrial carnitine palmitoyltransferase I (CPT I; EC 2.3.1.21) to inhibition by malonyl CoA in rat heart and skeletal muscle. The nutritional state of rats has been shown to have less influence on the activity and metabolic control of mitochondrial CPT I in heart and skeletal muscle tissue than in the liver, a tissue in which CPT I activity and sensitivity to inhibition by malonyl CoA can be shown to be regulated acutely under different nutritional conditions. However, because manipulation of the nutritional state in these previous studies was restricted mainly to examining the effect of starvation, this study was undertaken to determine whether, as in liver, the fatty acid content and composition of the diet can regulate the activity and metabolic control of CPT I in heart and skeletal muscle. Rats were fed for up to 10 wk either a nonpurified low fat diet (30 g fat/kg) or a high fat diet (200 g fat/kg) containing one of the following five oil types: hydrogenated coconut oil (HCO), olive oil (OO), safflower oil (SO), evening primrose oil (EPO) or menhaden (fish) oil (MO). Feeding a diet enriched in MO had the most pronounced effect. Rats fed MO had a significantly greater skeletal muscle CPT I specific activity and tissue capacity, and a lower sensitivity of CPT I to malonyl CoA inhibition compared with rats fed a low fat diet, but the duration of feeding required to modulate this sensitivity was longer than that observed previously for the liver enzyme. Progressively greater sensitivity of heart CPT I to malonyl CoA occurred with feeding duration in all groups. These studies indicate that the fatty acid composition of the diet is involved in the regulation of mitochondrial CPT I activity in heart and skeletal muscle.

Influence of diet on the kinetic behavior of hepatic carnitine palmitoyltransferase I toward different acyl CoA esters

The influence of diet on the kinetics of the overt form of rat liver mitochondrial carnitine palmitoyltransferase (CPT I; EC 2.3.1.21) was studied using rats fed either a low-fat diet (3% w/w fat), or diets which were supplemented with either olive oil (OO), safflower oil (SO) or menhaden (fish) oil (MO) to 20% w/w of fat (high fat diets). When animals were fed each of these four diets for 10 days, the order of the apparent maximal activity (Vmax) of CPT I toward various individual fatty acyl CoA, when measured under a fixed molar ratio of acyl CoA/albumin, was 16:1 n-7 > 18:1 n-9 > 18:2 n-6 > 16:0 > 22:6 n-3, and was thus not affected by the fat composition of the diet. However, in all but one case, the SO and MO diets elicited a higher Vmax for each substrate than either the LF diet or the high fat OO diet. The apparent K0.5 for the different acyl CoA esters was generally lowest in LF-fed animals, and highest in those fed the high-fat SO diet. Moreover, when compared with the situation of animals fed high-fat diets, the K0.5 values of CPT I in LF-fed animals for palmitoyl CoA and oleoyl CoA were low. This possession by CPT I of a high "affinity" toward these nonessential fatty acyl CoAs, but a lower "affinity" toward linoleoyl CoA, the ester of an essential fatty acid, may enable this latter fatty acid to be spared from oxidation when its concentration in the diet is low. The data also emphasize that palmitoleoyl CoA, if available in the diet, is likely to be utilized by CPT I at a high rate.

No correlation between changes in fatty acid-binding protein content and fatty acid oxidation capacity of rat tissues in experimental diabetes

Fatty acid-binding protein is considered to play an important role in fatty acid oxidation. Since diabetes mellitus causes marked changes of this latter metabolic process, we compared the effect of this pathological condition on both parameters in a comparative investigation of different rat tissues. Palmitate oxidation capacity and content of fatty acid-binding protein were determined in liver, heart and quadriceps muscle from rats with 2-week streptozotocin-induced diabetes mellitus and controls. In liver homogenates fatty acid oxidation capacity increased by 90%, but their content of fatty acid-binding protein decreased by 35%. Fatty acid oxidation capacity of heart and quadriceps muscle and fatty acid-binding protein content of quadriceps muscle did not change, but fatty acid-binding protein content of heart muscle doubled. Long-term diabetes (8 months) had a similar effect on content of this protein. In summary, changes of fatty acid oxidation capacity do not appear to correlate with fatty acid-binding protein content during the development of diabetes. This does not preclude other functions of fatty acid-binding proteins in regulation of lipid metabolism and processes in which fatty acids play a modulatory role.

A study of properties and abundance of the components of liver carnitine palmitoyltransferases in mitochondrial inner and outer membranes. Effects of hypothyroidism, fasting and a ketotic diabetic state

1. Liver mitochondrial outer and inner membranes were isolated from normal, 48 h-fasted, streptozotocin-diabetic and hypothyroid rats. 2. Relative to membrane protein, fasting and diabetes substantially increased the activity of carnitine palmitoyltransferase (CPT) in outer membranes. Inner-membrane CPT specific activity was only slightly altered, being increased in diabetes and decreased in hypothyroidism. Abundance of an inner-membrane Mr-68,000 polypeptide that cross-reacted with an anti-CPT serum was significantly increased in diabetes and hypothyroidism. Relative to inner-membrane CPT activity, this cross-reactivity was increased by 37% in diabetes and by 400% in hypothyroidism, suggesting modification of the intrinsic activity of the CPT in these states. 3. CPT in outer membranes was inhibitable by malonyl-CoA, whereas inner-membrane CPT was insensitive to malonyl-CoA. Fasting and diabetes increased the IC50 (concentration of malonyl-CoA causing 50% inhibition) for outer-membrane CPT, whereas the IC50 was decreased in hypothyroidism. 4. Binding of [14C]malonyl-CoA was observed with both outer and inner membranes and was fitted to two-site models in each case. Fasting, diabetes and hypothyroidism changed the KD for binding at the higher-affinity site in outer membranes in a manner that correlated closely with changes in IC50 for inhibition of outer-membrane CPT by malonyl-CoA. Fasting and diabetes increased the abundance of this outer-membrane high-affinity malonyl-CoA-binding site, whereas hypothyroidism decreased its abundance.

Carnitine palmitoyltransferase in cardiac ischemia. A potential site for altered fatty acid metabolism

The sensitivity of carnitine palmitoyl coenzyme A (CoA) transferase I to inhibition of its activity by malonyl-CoA is progressively reduced in mitochondria isolated from ischemic cardiac cells as blood flow decreases to 30% or less of the preocclusion flow. The activity of carnitine palmitoyl-CoA transferase I in mitochondria isolated from nonischemic cardiac cells demonstrates incomplete inhibition, even at high concentrations of malonyl-CoA. Kinetic analyses of these data gave results most consistent with the expression of two overt enzyme activities: one activity that is sensitive to inhibition by malonyl-CoA and one activity that demonstrates little or no sensitivity to such inhibition. The decrease in malonyl-CoA-sensitive activity associated with ischemia results from a 13% decrease in the activity of the sensitive component and a corresponding 13% increase in the activity of the insensitive component. Decreased sensitivity of ischemic carnitine palmitoyl-CoA transferase I to inhibition by malonyl-CoA, together with potential fluctuations in the content of malonyl-CoA in tissue, would increase the synthesis of palmitoylcarnitine during ischemia and facilitate return to the use of fatty acid as a preferred metabolic fuel on reperfusion. This apparent conversion occurs concomitantly with a decrease in the free protein thiol content of the mitochondrial membranes isolated from ischemic cardiac cells. Treatment of the mitochondria from ischemic cardiac cells with dithiothreitol in vitro partially reverses the loss in sensitivity to malonyl-CoA, suggesting the possible role of thiol oxidation in the altered metabolism of ischemic mitochondria. Western blot analysis of these mitochondria using an antibody against carnitine palmitoyltransferase II purified from beef heart demonstrates a 68-kDa protein, which under ischemic conditions apparently is decreased by 2 kDa. These results are more indicative of a modification in protein folding of carnitine palmitoyltransferase than proteolytic changes during ischemia.

The effect of palmitoyl-CoA binding to albumin on the apparent kinetic behavior of carnitine palmitoyltransferase I

Substrate saturation plots of carnitine palmitoyltransferase I activity from isolated rat liver mitochondria vs. palmitoyl-CoA concentration in the presence of bovine serum albumin have been reported to yield sigmoidal kinetics. Under identical assay conditions we have confirmed these observations as reflected by nonlinear Lineweaver-Burke plots (1/vi vs. 1/[S]) an average Hill coefficient of napp. = 1.98 +/- 0.09 (Mean +/- S.E. from four separate experiments). For these determinations the enzyme activity was plotted against the total [palmitoyl-CoA] in the presence of 0.13% bovine serum albumin. Utilizing the total [palmitoyl-CoA] to determine the kinetic properties of carnitine palmitoyltransferase I would be valid only if the relationship between total and free [palmitoyl-CoA] was linear, which is not the case as we have previously shown. When carnitine palmitoyltransferase I substrate saturation kinetics were reanalyzed using the previously determined free [palmitoyl-CoA]'s, the plots revealed a shift to standard hyperbolic kinetics. This observation was confirmed by an average Hill coefficient of napp. = 1.04 +/- 0.10 (Mean +/- S.E.) and linear Lineweaver-Burke plots. The double-reciprocal plots from these analyses yielded an average S0.5 of 2.55 +/- 0.82 microM (Mean +/- S.E.) palmitoyl-CoA and Vmax of 19.69 +/- 5.48 nmol/min per mg protein. These studies clearly demonstrate the importance of defining the free [palmitoyl-CoA] when analyzing the kinetics of carnitine palmitoyltransferase I in the presence of bovine serum albumin.

The binding of palmitoyl-CoA to bovine serum albumin

Bovine serum albumin (BSA) is routinely utilized in vitro to prevent the adverse detergent effects of long-chain acyl-CoA esters (i.e., palmitoyl-CoA) in enzyme assays. Determination of substrate saturation kinetics in the presence of albumin would only be valid if the relationship between bound and free substrate concentrations was known. To elucidate the relationship between bound and free palmitoyl-CoA concentrations in the presence of BSA, several different techniques including equilibrium dialysis, equilibrium partitioning, fluorescence polarization and direct fluorescence enhancement were investigated. Direct fluorescence enhancement using a custom synthesized fluorescent probe, 16-(9-anthroyloxy)palmitoyl-CoA (AP-CoA), was the best approach to this question. Measurement of the relationship between mol of palmitoyl-CoA bound per mol of BSA (nu) versus -log[free palmitoyl-CoA] revealed that the binding of palmitoyl-CoA to BSA, like palmitate was nonlinear, suggesting the presence of more than one class of acyl-CoA binding sites. Computer analyses of the binding data gave a best fit to the 2,4 two-class Scatchard model, suggesting the presence of two high-affinity primary binding sites (k1 = (1.55 +/- 0.46) x 10(-6) M-1) and four lower affinity secondary binding sites (k2 = (1.90 +/- 0.09) x 10(-8) M-1). Further analyses using the six parameter stoichiometric (stepwise) ligand binding model supports the existence of six binding sites with the higher affinities associated with the binding of the first mole of palmitoyl-CoA and weaker binding occurring after the first two sites are occupied. The association constants from this model of multiple binding diminish sequentially (i.e., K1 greater than K2 greater than K3 greater than...greater than or equal to K6), suggesting that each mol of long-chain acyl-CoA binds to BSA with decreasing affinities.

Turnover of carnitine palmitoyltransferase mRNA and protein in H4IIE cells. Effect of cyclic AMP and insulin

Regulation of the 68 kDa carnitine palmitoyltransferase (CPT) synthesis by (chlorophenylthio) cyclic AMP (cAMP) and insulin was studied in H4IIE cells in culture. Addition of 0.1 mM- or 1.0 mM-(chlorophenylthio) cAMP induced CPT mRNA and rate of transcription 2-4-fold by 15 min, reaching a plateau at 4-6-fold by 30 min. Addition of 5-15 nM-insulin plus 1.0 mM-cAMP suppressed the increases in transcription rate and mRNA levels occurring with cAMP alone. The t1/2 for CPT mRNA was 70-80 min and was not affected by cAMP. The t1/2 for CPT protein was 70 min, and was increased to 240 min in the presence of cAMP. The rate of CPT synthesis was also increased in the presence of cAMP. The data indicate that CPT synthesis is increased by cAMP via induction of transcription and subsequent increase in the CPT mRNA. Insulin acts to depress transcription and CPT mRNA. In addition, cAMP prolongs the t1/2 of CPT.

Regulation of carnitine palmitoyltransferase in vivo by glucagon and insulin

Carnitine palmitoyltransferase (CPT total) activity and synthesis increase in states where the insulin/glucagon ratio is low, such as starvation and diabetes [Brady & Brady (1987) Biochem. J. 246, 641-646]. However, the effect of glucagon and insulin on CPT synthesis is unknown. The present experiments were designed to determine the effect of glucagon, cAMP [8-(chlorophenylthio) cyclic AMP], and insulin + cAMP on CPT transcription and mRNA amounts over time after injection. The CPT protein that was purified, used to generate antibody, and cloned in these studies was the 68 kDa mitochondrial protein described previously [Brady & Brady (1987) Biochem. J. 246, 641-646; Brady, Feng & Brady (1988) J. Nutr. 118, 1128-1136; Brady & Brady (1989) Diabetes 38, in the press]. Saline-injected control rats exhibited a 2-fold increase in hepatic CPT transcription rate and CPT mRNA over the 5 h experiment from 09:00 to 14:00 h. The effect was most probably due to the fasting state of the rats during the day. Glucagon injection caused an 8-fold increase in transcription rate by 90 min and a 4-fold increase in CPT mRNA by 90-120 min. The cAMP effect had reached a peak by the first time point taken (15 min). Transcription rate was increased 4-fold and CPT mRNA was increased 3-fold at this time. The combination of cAMP + insulin injection did not produce any significant increase in transcription rate or CPT mRNA over the saline-injected controls. CPT mRNA and transcription rate showed a clear dose-response to glucagon injection from 0 to 150 micrograms/100 g body wt. Total CPT activity and immunoreactive CPT were not increased during these experiments. The data indicate that glucagon and insulin interact in control of transcription rate and amount of CPT mRNA, but that increases in CPT immunoreactive protein and activity are temporally delayed. This lag probably relates to the half-life of the CPT protein in vivo, which has been estimated as 2-7 days.

Rapid effects of insulin and glucose on the hepatic incorporation of gluconeogenic substrates into glyceride glycerol and glycogen

1. The hepatic utilization of gluconeogenic substrates was investigated shortly after portal infusion of either insulin or glucose in fasted rats. 2. After 20 min of insulin infusion blood glucose concentration decreased. However, neither glucose generation from precursors such as alanine or pyruvate nor their incorporation into fatty acids was modified. Under these conditions, insulin rapidly increased the incorporation of gluconeogenic substrates into the hepatic glyceride glycerol fraction. Insulin treatment led to a decrease in substrate incorporation into liver glycogen. 3. After 20 min of portal glucose infusion both plasma insulin and glucose concentrations increased and the incorporation of pyruvate into hepatic glyceride glycerol and into glycogen was also stimulated. 4. A close relationship was observed between blood glucose concentrations and the level of incorporation of gluconeogenic substrates into liver glycogen. 5. In conclusion, during fasting insulin stimulates the incorporation of gluconeogenic substrates into the glycerol moiety of hepatic glycerides, which may be the preferential mechanism through which fatty acid esterification is accomplished during refeeding. This effect of insulin is rapid and detected even before other classical modifications induced by the hormone such as gluconeogenesis inhibition or lipogenesis activation. Furthermore, the effect is not related to insulin-induced hypoglycemia since glucose infusion mimics insulin action on glyceride glycerol synthesis.

Hemipalmitoylcarnitinium, a strong competitive inhibitor of purified hepatic carnitine palmitoyltransferase

We have synthesized (2S,6R:2R,6S)-6-carboxymethyl-2-hydroxy-2-pentadecyl-4,4-dimethylmorp holinium bromide (hemipalmitoylcarnitinium, HPC) which is a conformationally restricted analog inhibitor of carnitine palmitoyltransferase (CPT; EC 2.3.1.21). rac-HPC inhibits catalytic activity in purified rat liver CPT. In the forward reaction, HPC competes with both (R)-carnitine (Ki(app) = 5.1 +/- 0.7 microM) and palmitoyl-CoA (Ki(app) = 21.5 +/- 4.9 microM). In the reverse reaction, inhibition by HPC is competitive with palmitoyl-(R)-carnitine (Ki(app) = 1.6 +/- 0.6 microM), but inhibition is uncompetitive with CoA. The forward reaction is also competitively inhibited by its product, palmitoyl-(R)-carnitine, Ki(app)'s 14.2 +/- 2.1 microM relative to (R)-carnitine and 8.7 +/- 2.6 microM relative to palmitoyl-CoA. rac-HPC is the most potent synthetic reversible inhibitor of purified CPT. HPC fails to inhibit carnitine acetyltransferase (CAT; EC 2.3.1.7). Palmitoylcholine also inhibits CPT in the forward reaction, competing with (R)-carnitine (Ki(app) = 18.6 +/- 4.5 microM) and with palmitoyl CoA (Ki(app) = 10.4 +/- 2.5 microM). Choline is not an effective CPT inhibitor. We have shown [R.D. Gandour et al. (1986) Biochem. Biophys. Res. Commun. 138, 735-741] that hemiacetylcarnitinium inhibits CAT but not CPT. The combined data demonstrate further differences between the carnitine recognition sites in CPT and CAT.

Changes in brown-adipose-tissue mitochondrial processes in streptozotocin-diabetes

Diabetic rats were used as a source of brown-adipose-tissue mitochondria 2 days after a single subcutaneous injection of streptozotocin (100 mg/kg). Diabetes caused an 80% decrease in carnitine-dependent oxidation of palmitoyl-CoA and a 50-60% decrease in overt carnitine palmitoyltransferase activity. An additional lesion in brown-adipose-tissue mitochondrial oxidative capacity was also indicated, since diabetes increased by 30-50% the rate of oxidation under uncoupled conditions of several respiratory substrates (i.e. malate + palmitoylcarnitine, malate + pyruvate, succinate, NNN'N'-tetramethyl-p-phenylenediamine + ascorbate). This decrease in mitochondrial function was accompanied by an approx. 30% decrease in the abundance of cytochromes (a + a3) and total cytochromes b.

Hepatic carnitine palmitoyltransferase turnover and translation rates in fed, starved, streptozotocin-diabetic and diethylhexyl phthalate-treated rats

Hepatic carnitine palmitoyltransferase (CPT) turnover was studied in control and in non-ketotic hyperglycaemic streptozotocin-diabetic rats. The degradation constant (kd) and half-life (t1/2) did not appear to be altered by mild diabetes. The hepatic CPT (micrograms/g of liver) was not increased by the mild, non-ketotic, diabetes. However, the total hepatic CPT (micrograms/liver) was 37% greater in the diabetic animals, owing to the increased liver weight. This resulted from a 40% increase in the synthesis constant (ks). Hepatic CPT activity (total detergent-solubilized) and translation rates were measured in fed, starved (48 h), non-ketotic diabetic, ketotic diabetic and diethylhexyl phthalate (DEHP)-treated rats. CPT activity (m units/mg of mitochondrial protein) was not significantly increased with non-ketotic diabetes (44% increase, but non-significant), but was increased approx. 2-fold with starvation and ketotic diabetes, and 3.5-fold with DEHP treatment. CPT expressed as units/liver was increased non-significantly (23%) in non-ketotic and starved rats, similar to the turnover study, but was significantly increased with ketotic diabetes and with DEHP treatment. mRNA-translation activity for CPT was elevated in all states to a somewhat greater extent than was activity. It was concluded that protein synthesis as a product of increased CPT-mRNA translation activity is a major means of long-term regulation.

Regulation of fatty acid oxidation in isolated hepatocytes and liver mitochondria from newborn rabbits

The changes in long-chain fatty acid oxidation during the first 24 h after birth were studied in isolated rabbit hepatocytes and liver mitochondria. The eightfold increase in this oxidation which occurs in hepatocytes between birth and 24 h was not triggered by a concomitant decrease in long-chain fatty acid esterification. Indeed, in isolated hepatocytes from 24-h-old rabbits, the 75% inhibition of the oxidation by 2-tetradecylglycidic acid, resulted in a total redirection of oleate metabolized towards triacylglycerol synthesis. Polarographic measurements of mitochondrial respiration showed that oxidative phosphorylation and respiratory chain capacity were fully functional at birth. By contrast, in liver mitochondria isolated from newborn rabbits, the rate of oxygen consumption from palmitoyl-L-carnitine was 60% higher than from palmitoyl-CoA. Similarly palmitoyl-CoA oxidation was increased 1.5-fold in isolated mitochondria from 24-h-old rabbits. These results were in agreement with the twofold increase in the activity of hepatic carnitine palmitoyltransferase I between birth and 24 h. However it is unlikely that the twofold increase in this enzyme activity totally explained the eightfold increase in long-chain fatty acid oxidation in isolated newborn rabbit hepatocytes. It was shown that the rate of the oxidation in isolated hepatocytes was inversely related to the rate of lipogenesis. Nevertheless, malonyl-CoA concentration per se is probably not the factor involved in the regulation of the oxidation between birth and 24 h, since a 90% decrease in hepatic malonyl-CoA concentration was not associated with a stimulation of long-chain fatty acid oxidation. The more likely mechanism was the 30-fold decrease in the sensitivity of carnitine palmitoyltransferase I to malonyl-CoA inhibition.

Carnitine palmitoyltransferase in liver and five extrahepatic tissues in the rat. Inhibition by DL-2-bromopalmitoyl-CoA and effect of hypothyroidism

Mitochondria were isolated from rat adult liver, foetal liver, kidney cortex, heart, skeletal muscle and interscapular brown adipose tissue. DL-2-Bromopalmitoyl-CoA inhibited the overt form of carnitine palmitoyltransferase (CPT1) in heart, skeletal muscle and brown adipose tissue, with an IC50 value (concentration giving 50% inhibition) of 1.3-1.6 microM. By contrast, the IC50 value for inhibition of the kidney or adult liver enzyme was 0.08-0.1 microM. CPT1 in near-term foetal liver differed from that in adult liver in that the IC50 for inhibition by 2-bromopalmitoyl-CoA was 0.57 microM. It is suggested that there may be tissue-specific forms of the catalytic entity of CPT1 and that foetal liver may contain a mixture of adult liver- and muscle-type enzymes. In rats made hypothyroid by administration of propylthiouracil and an iodine-deficient diet, hepatic CPT1 activity was decreased by 83%. However, CPT1 activity in extrahepatic tissues showed no adaptive decrease in hypothyroidism.

Hepatic mitochondrial inner-membrane properties, beta-oxidation and carnitine palmitoyltransferases A and B. Effects of genetic obesity and starvation

Hepatic mitochondrial carnitine palmitoyltransferase (CPT) properties, beta-oxidation of palmitoyl-CoA and membrane polarization were measured in lean and obese Zucker rats. The Vmax. of the 'outer' carnitine palmitoyltransferase ('CPT-A') increased with starvation, with no change in the Km for either carnitine or palmitoyl-CoA. The Ki for malonyl-CoA increased with starvation in lean rats, but not in obese rats. The Vmax. of the 'inner' enzyme ('CPT-B'), as measured by using inverted submitochondrial vesicles, increased with starvation in obese rats only, with no change in the Km for either carnitine or palmitoyl-CoA. The Ki for malonyl-CoA was 2-5-fold higher in inverted vesicles than in intact mitochondria, and showed no alteration with starvation. The activities of both enzymes correlated positively with each other and with beta-oxidation, and inversely with membrane polarization. Malonyl-CoA had little effect on gross membrane fluidity in the Zucker rat, as reflected by diphenylhexatriene fluorescence polarization. The results indicate that both enzymes are related and respond similarly to alterations in membrane fluidity. Membrane fluidity may provide a mechanism for co-ordinated control of CPT activity on both sides of the mitochondrial inner membrane.

Hepatic mitochondrial inner membrane properties and carnitine palmitoyltransferase A and B. Effect of diabetes and starvation

Intact mitochondria and inverted submitochondrial vesicles were prepared from the liver of fed, starved (48 h) and streptozotocin-diabetic rats in order to characterize carnitine palmitoyltransferase kinetics and malonyl-CoA sensitivity in situ. In intact mitochondria, both starved and diabetic rats exhibited increased Vmax., increased Km for palmitoyl-CoA, and decreased sensitivity to malonyl-CoA inhibition. Inverted submitochondrial vesicles also showed increased Vmax. with starvation and diabetes, with no change in Km for either palmitoyl-CoA or carnitine. Inverted vesicles were uniformly less sensitive to malonyl-CoA regardless of treatment, and diabetes resulted in a further decrease in sensitivity. In part, differences in the response of carnitine palmitoyltransferase to starvation and diabetes may reside in differences in the membrane environment, as observed with Arrhenius plots, and the relation of enzyme activity and membrane fluidity. In all cases, whether rats were fed, starved or diabetic, and whether intact or inverted vesicles were examined, increasing membrane fluidity was associated with increasing activity. Malonyl-CoA was found to produce a decrease in intact mitochondrial membrane fluidity in the fed state, particularly at pH 7.0 or less. No effect was observed in intact mitochondria from starved or diabetic rats, or in inverted vesicles from any of the treatment groups. Through its effect on membrane fluidity, malonyl-CoA could regulate carnitine palmitoyltransferase activity on both surfaces of the inner membrane through an interaction with only the outer surface.

Inhibitory effects of some long-chain unsaturated fatty acids on mitochondrial beta-oxidation. Effects of streptozotocin-induced diabetes on mitochondrial beta-oxidation of polyunsaturated fatty acids

Evidence showing that some unsaturated fatty acids, and in particular docosahexaenoic acid, can be powerful inhibitors of mitochondrial beta-oxidation is presented. This inhibitory property is, however, also observed with the cis- and trans-isomers of the C18:1(16) acid. Hence it is probably the position of the double bond(s), and not the degree of unsaturation, which confers the inhibitory property. It is suggested that the inhibitory effect is caused by accumulation of 2,4-di- or 2,4,7-tri-enoyl-CoA esters in the mitochondrial matrix. This has previously been shown to occur with these fatty acids, in particular when the supply of NADPH was limiting 2,4-dienoyl-CoA reductase (EC 1.3.1.-) activity [Hiltunen, Osmundsen & Bremer (1983) Biochim. Biophys. Acta 752, 223-232]. Liver mitochondria from streptozotocin-diabetic rats showed an increased ability to beta-oxidize 2,4-dienoyl-CoA-requiring acylcarnitines. Docosahexaenoylcarnitine was also found to be less inhibitory at lower concentrations with incubation under coupled conditions. With uncoupling conditions there was little difference between mitochondria from normal and diabetic rats in these respects. This correlates with a 5-fold stimulation of 2,4-dienoyl-CoA reductase activity found in mitochondria from streptozotocin-diabetic rats.

Cycloheximide blocks changes in rat liver carnitine palmitoyltransferase 1 activity in starvation

Starvation (24h) increased the maximum activity of carnitine palmitoyltransferase 1 in rat liver and increased the concentration of malonyl-CoA required to cause 50% inhibition of the enzyme (I50). Re-feeding (24h) with a standard cube diet or a high-carbohydrate diet reversed both of these changes, whereas re-feeding with a high-fat diet did not. Administration of cycloheximide (200 micrograms/100 g body wt.) blocked the increases in carnitine palmitoyltransferase 1 activity and I50 on starvation. It is suggested that increase in carnitine palmitoyltransferase 1 activity in starvation may involve synthesis of new enzyme.

Carnitine palmitoyltransferase I. Inhibition by D-galactosamine and role of phospholipids

Palmitate oxidation by liver mitochondria from rats treated with D-galactosamine (GalN) was markedly inhibited, 3 h after administration. The mitochondrial defect responsible for this inhibition was shown to be an inhibition of the activity of palmitoylcarnitine transferase I (EC 2.3.1.21). Apparent Km of the enzyme remained unchanged whereas apparent V was reduced by 30%. Addition of 10 mM GalN did not impair the activity of palmitoylcarnitine transferase I in mitochondria isolated from normal rats. Inhibition of palmitoylcarnitine biosynthesis by GalN treatment was completely reversed by phospholipid supply. At this stage of intoxication, mitochondrial phospholipid content was decreased whereas incorporation of [14C]palmitate into phospholipids in isolated hepatocytes was drastically inhibited: the phosphatidylcholine/phosphatidylethanolamine ratio was reduced by 33%. The results obtained from these studies show that the depletion of the phospholipid membrane content could account for the altered functional activity of palmitoylcarnitine transferase I.

Mechanism of adrenergic stimulation of hepatic ketogenesis

The effects of alpha- and beta-adrenergic stimulation on ketogenesis were examined in freshly isolated rat hepatocytes in order to determine which alpha- or beta-adrenergic stimulation is involved in the enhancement of ketogenesis. In the presence of 0.3 mmol/L (U-14C)-palmitate, epinephrine, norepinephrine, and phenylephrine at 500 ng/mL increased ketogenesis by 25% (16.0 +/- 0.17 v 12.8 +/- 0.13 nmol/mg protein per hour), 20% (15.3 +/- 0.28) and 20% (15.4 +/- 0.36), respectively. However, isoproterenol even at 1 microgram/mL did not stimulate ketogenesis. Phentolamine (5 micrograms/mL) almost completely abolished the effect of epinephrine on ketogenesis (13.7 +/- 0.30 v 16.0 +/- 0.17) but propranolol did not inhibit the stimulation by epinephrine (15.6 +/- 0.38 v 16.0 +/- 0.17). Trifluoperazine (10 mumol/L), presumably an inhibitor of calcium-dependent protein kinase, abolished the effect of epinephrine (13.6 +/- 0.22 v 16.0 +/- 0.17). These results indicate that catecholamines increase ketogenesis predominantly through the alpha-adrenergic system independent of cyclic AMP, and calcium-dependent protein kinase is thought to be involved in the activation of ketogenesis. On the other hand, glucagon stimulated ketogenesis with an increase of cyclic AMP, which was not inhibited by alpha- and beta-adrenergic antagonists. Alpha-adrenergic stimulation increased hepatic glycogenolysis much more at much lower concentrations when compared with ketogenesis. Stimulation of ketogenesis by catecholamines seemed to be less sensitive and responsive compared with hepatic glycogenolysis.

The effect of malonyl-CoA on overt and latent carnitine acyltransferase activities in rat liver and adipocyte mitochondria

1. Carnitine palmitoyltransferase and carnitine octanoyltransferase activities were measured in mitochondria at various acyl-CoA concentrations before and after sonication, thus permitting assessment of both overt and latent activities. 2. Overt carnitine palmitoyltransferase in liver and adipocyte mitochondria and overt carnitine octanoyltransferase in liver mitochondria were inhibited by malonyl-CoA. None of the latent activities were affected by this metabolite. 3. 5,5'-Dithiobis-(2-nitrobenzoic acid) stimulated latent hepatic carnitine palmitoyltransferase at low [palmitoyl-CoA]. 4. Starvation (24 h) decreased overt carnitine palmitoyltransferase activity in adipocyte mitochondria, but did not alter the sensitivity of this activity to malonyl-CoA.

Response to starvation of hepatic carnitine palmitoyltransferase activity and its regulation by malonyl-CoA. Sex differences and effects of pregnancy

1. Hepatic carnitine palmitoyltransferase activity was measured over a range of concentrations of palmitoyl-CoA and in the presence of several concentrations of the inhibitor malonyl-CoA. These measurements were made in mitochondria obtained from the livers of fed and starved (24 h) virgin female and fed and starved pregnant rats. 2. In the fed state overt carnitine palmitoyltransferase activity was significantly lower in virgin females than in age-matched male rats. 3. Starvation increased overt carnitine palmitoyltransferase activity in both virgin and pregnant females. This increase was larger than in the male and was greater in pregnant than in virgin females. 4. In the fed state pregnancy had no effect on the Hill coefficient or the [S]0.5 when palmitoyl-CoA was varied as substrate. Pregnancy did not alter the sensitivity of the enzyme to inhibition by malonyl-CoA. 5. Starvation decreased the sensitivity of the enzyme to malonyl-CoA. The change in sensitivity was similar in male, virgin female and pregnant rats. 6. The possible relevance of these findings to known sex differences and changes with pregnancy in hepatic fatty acid oxidation and esterification are discussed.

Carnitine acyltransferase activities in rat liver and heart measured with palmitoyl-CoA and octanoyl-CoA. Latency, effects of K+, bivalent metal ions and malonyl-CoA

1. Liver carnitine acyltransferase activities with palmitoyl-CoA and octanoyl-CoA as substrates and heart carnitine palmitoyltransferase were measured as overt activities in whole mitochondria or in mitochondria disrupted by sonication or detergent treatment. All measurements were made in sucrose/KCl-based media of 300 mosmol/litre. 2. In liver mitochondria, acyltransferase measured with octanoyl-CoA, like carnitine palmitoyltransferase, was found to have latent and overt activities. 3. Liver acyltransferase activities measured with octanoyl-CoA and palmitoyl-CoA differed in their response to changes in [K+], Triton X-100 treatment and, in particular, in their response to Mg2+. Mg2+ stimulated activity with octanoyl-CoA, but inhibited carnitine palmitoyltransferase. 4. The effects of K+ and Mg2+ on liver overt carnitine palmitoyltransferase activity were abolished by Triton X-100 treatment. 5. Heart overt carnitine palmitoyltransferase activity differed from the corresponding activity in liver in that it was more sensitive to changes in [K+] and was stimulated by Mg2+. Heart had less latent carnitine palmitoyltransferase activity than did liver. 6. Overt carnitine palmitoyltransferase in heart mitochondria was extremely sensitive to inhibition by malonyl-CoA. Triton X-100 abolished the effect of low concentrations of malonyl-CoA on this activity. 7. The inhibitory effect of malonyl-CoA on heart carnitine palmitoyltransferase could be overcome by increasing the concentration of palmitoyl-CoA.

Activation and transport of fatty acids in ovarian mitochondria: effect of Lh

Enzymes of fatty acid activation and transport were studied in luteinized rat ovaries. Luteal mitochondria were found to contain high levels of palmitoyl-CoA synthetase and carnitine palmitoyl-transferase activities. In addition, studies on the effect of palmitate concentration on palmitoyl-CoA synthetase activity revealed the possible existence of two forms of the enzyme: Km values of 0.34 mM and 21.33 mM, with Vmax of 3.64 and 66.67 nmoles/min/mg mitochondrial protein respectively, were obtained for the two activities. Similar kinetic data for carnitine palmitoyl-transferase activity in intact mitochondria are a Km of 21 microM and a Vmax of 18.2 nmoles/min/mg mitochondrial protein. Only one activity of this enzyme could be detected in luteal mitochondria. It appears that the activities of both enzymes were not affected by prior administration of LH in vivo. The possibility that this negative finding was due to the experimental procedures employed, rather than a reflection of the situation in vivo, could not be discounted, although its more likely that these two enzymes are probably not locus of LH stimulation. The results indicate that fatty acid oxidation is an important metabolic capability of luteal mitochondria, and support the view regarding the lipid nature of the respiratory fuel of ovarian tissue.

The effect of glucagon treatment and starvation of virgin and lactating rats on the rates of oxidation of octanoyl-L-carnitine and octanoate by isolated liver mitochondria

1. Oxygen-consumption rates owing to oxidation of octanoate or octanoylcarnitine by isolated mitochondria from livers of fed, starved and glucagon-treated virgin or 12-day-lactating animals were measured under State-3 and State-4 conditions, in the presence or absence of l-malate and inhibitors of tricarboxylic acid-cycle activity (malonate and fluorocitrate). 2. Mitochondria from fed lactating animals had a slightly lower rate of octanoylcarnitine oxidation than did those of fed virgin animals, whereas the rates of octanoate oxidation were unaffected. 3. Starvation of virgin animals for 24h or 48h resulted in a large (70-100%) increase in mitochondrial octanoylcarnitine oxidation; rates of octanoate oxidation were either unaffected (24 and 48h starvation in the absence of malonate and fluorocitrate) or diminished by 30% (48h starvation in the presence of inhibitors). In lactating animals, 24h starvation resulted in a smaller increase in the rate of octanoylcarnitine oxidation than that obtained for mitochondria from virgin rats. 4. Glucagon treatment (by intra-abdominal injection) of fed virgin and lactating rats increased the rate of mitochondrial oxidation of both octanoylcarnitine and octanoate. Injection of glucagon into 48h-starved virgin rats did not increase further the already elevated rate of octanoylcarnitine oxidation, but reversed the inhibition of octanoate beta-oxidation observed for these mitochondria in the presence of malonate and fluorocitrate. 5. It is suggested that glucagon activates octanoylcarnitine oxidation by increasing the activity of the carnitine/acylcarnitine transport system [Parvin & Pande (1979) J. Biol. Chem.254, 5423-5429] and that the increase in octanoate oxidation by mitochondria from glucagon-treated animals is caused by the increased rate of ATP synthesis in these mitochondria. 6. The results are discussed in relation to the increased capacity of the liver to oxidize long-chain fatty acids and carnitine esters of medium-chain fatty acids under conditions characterized by increased ketogenesis.

[Clinical, morphological and biochemical studies on muscle carnitine deficiency (author's transl)]

This report deals with two sisters who died with eight, respectively ten weeks under the signs of respiratory failure caused by progressive muscular weakness. Only an elevated cerebrospinal fluid protein was suspicious of an additional disturbance of the central nervous system. Muscle biopsy revealed a vacuolar myopathy. Histochemistry showed lipid storage, increased mitochondrial enzyme activity, and to a lower degree, glycogen accumulation especially in type I muscle fibers. Electron microscopy confirmed elevated lipid content in combination with increased, enlarged and abnormally structured mitochondria. Biochemical studies on muscle biopsy, in comparison with normal children, showed a significant decrease of carnitine content and an increased activity of carnitine palmityltransferase. Retrospectively from a clinical point of view this disease is suggestive of "systemic carnitine deficiency", even if some symptoms (hepatomegaly, cardiomyopathy) were not present and serum- and liver carnitine was not measured because the children died before the diagnosis of muscle carnitine deficiency was confirmed. The clinical picture of these two fatal cases is compared with another observation of muscle caritine deficiency. This child shows only a mild course of muscle disorder, but very similar morphological changes in muscle biopsy. Biochemically, there was a clear decrease in muscular carnitine, while the serum levels were in the normal range. The activity of muscular carnitine palmityltransferase was also normal.

The effect of palmitoyl-coenzyme A on rat heart and liver mitochondria. Oxygen consumption and palmitoylcarnitine formation

Rat heart and liver mitochondria, respectively, oxidized palmitoyl-CoA and palmitoylcarnitine optimally at 20-30 and 10-20 nmol substrate/mg. The oxidation of palmitoyl-CoA was accompanied by a lag in State 3 respiration that was proportional to the palmitoyl-CoA concentration. The delay in State 3 rates was more prolonged in liver than in heart at comparable palmitoyl-CoA levels. A similar range of palmitoyl-CoA concentrations produced significant inhibition of respiration in mitochondria oxidizing glutamate-malate. The inhibition was not due to a detergent effect of palmitoyl-CoA since addition of carnitine restored State 3 rates. Electron microscopic examination of mitochondria at low palmitoyl-CoA levels revealed normal ultrastructure. At comparable concentrations of palmitoyl-CoA, formation of palmitoylcarnitine by mitochondria from rat heart and liver followed first-order kinetics. The apparent first-order rate constants decreased with increasing palmitoyl-CoA. These results suggest that substrate inhibition may influence the rate of palmitoyl carnitine formation even at physiological concentrations of palmitoyl-CoA. The apparent first-order rate constant at palmitoyl-CoA levels (12 nmol palmitoyl CoA/mg) optimally oxidized by liver mitochondria, was one-third the value of the apparent rate constant measured in heart mitochondria at the identical substrate level. The prolongation in time to reach equilibrium may acocunt for the relatively greater respiratory sensitivity of liver mitochondria to increasing levels of palmitoyl-CoA.

Some aspects of fatty acid oxidation in isolated fat-cell mitochondria from rat

Mitochondrial were prepared from fat-cells isolated from rat epididymal adipose tissues of fed and 48 h-starved rats to study some aspects of fatty acid oxidation in this tissue. The data were compared with values obtained in parallel experiments with liver mitochondria that were prepared and incubated under identical conditions. 2. In the presence of malonate, fluorocitrate and arsenite, malate, but not pyruvate-bicarbonate, facilitated palmitoyl-group oxidation in both types of mitochondria. In the presence of malate, fat-cell mitochondria exhibited slightly higher rates of palmitoylcarnitine oxidation than liver. Rates of octanoylcarnitine oxidation were similar in liver and fat-cell mitochondria. Uncoupling stimulated acylcarnitine oxidation in liver, but not in fat-cell mitochondria. Oxidation of palmitoyl- and octanoyl-carnitine was partially additive in fat-cell but not in liver mitochondria. Starvation for 48 h significantly decreased both palmitoylcarnitine oxidation and latent carnitine palmitoyltransferase activity in fat-cell mitochondria. Starvation increased latent carnitine palmitoyltransferase activity in liver mitochondria but did not alter palmitoylcarnitine oxidation. These results suggested that palmitoylcarnitine oxidation in fat-cell but not in liver mitochondria may be limited by carnitine palmitoyltransferase 2 activity. 3. Fat-cell mitochondria also differed from liver mitochondria in exhibiting considerably lower rates of carnitine-dependent oxidation of palmitoyl-CoA or palmitate, suggesting that carnitine palmitoyltransferase 1 activity may severely rate-limit palmitoyl-CoA oxidation in adipose tissue.