Effects of DL-2-bromopalmitoyl-CoA and bromoacetyl-CoA in rat liver and heart mitochondria. Inhibition of carnitine palmitoyltransferase and displacement of [14C]malonyl-CoA from mitochondrial binding sites.

The overt form of carnitine palmitoyltransferase (CPT1) in rat liver and heart mitochondria was inhibited by DL-2-bromopalmitoyl-CoA and bromoacetyl-CoA. S-Methanesulphonyl-CoA inhibited liver CPT1. The inhibitory potency of DL-2-bromopalmitoyl-CoA was 17 times greater with liver than with heart CPT1. Inhibition of CPT1 by DL-2-bromopalmitoyl-CoA was unaffected by 5,5'-dithiobis-(2-nitrobenzoic acid) or (in liver) by starvation. In experiments in which DL-2-bromopalmitoyl-CoA displaced [14C]malonyl-CoA bound to liver mitochondria, the KD (competing) was 25 times the IC50 for inhibition of CPT1 providing evidence that the malonyl-CoA-binding site is unlikely to be the same as the acyl-CoA substrate site. Bromoacetyl-CoA inhibition of CPT1 was more potent in heart than in liver mitochondria and was diminished by 5,5'-dithiobis-(2-nitrobenzoic acid) or (in liver) by starvation. Bromoacetyl-CoA displaced bound [14C]malonyl-CoA from heart and liver mitochondria. In heart mitochondria this displacement was competitive with malonyl-CoA and was considerably facilitated by L-carnitine. In liver mitochondria this synergism between carnitine and bromoacetyl-CoA was not observed. It is suggested that bromoacetyl-CoA interacts with the malonyl-CoA-binding site of CPT1. L-Carnitine also facilitated the displacement by DL-2-bromopalmitoyl-CoA of [14C]malonyl-CoA from heart, but not from liver, mitochondria. DL-2-Bromopalmitoyl-CoA and bromoacetyl-CoA also inhibited overt carnitine octanoyl-transferase in liver and heart mitochondria. These findings are discussed in relation to inter-tissue differences in (a) the response of CPT1 activity to various inhibitors and (b) the relationship between high-affinity malonyl-CoA-binding sites and those sites for binding of L-carnitine and acyl-CoA substrates.     

The localization of palmitoyl-CoA: Carnitine palmitoyltransferase in rat liver

1. The distribution of palmitoyl-CoA:carnitine palmitoyltransferase has been studied in subcellular fractions of rat liver. By using two different estimations for the enzyme activity and by differential centrifugation and linear sucrose density gradient centrifugation, the enzyme is shown to be localized both in mitochondria and microsomes.

2. The mitochondrial palmitoyl-CoA: carnitine palmitoyltransferase is localized in the inner membrane plus matrix fraction.

3. During palmitate oxidation by isolated mitochondria, in the presence of a physiological concentration of carnitine, palmitoylcarnitine accumulates. From this and experiments with sonicated mitochondria, it is concluded that the capacities of long-chain fatty acid activation and of palmitoyl-CoA:carnitine palmitoyltransferase in vitro by far exceed the capacity of fatty acid oxidation.     

Carnitine palmitoyltransferase: separation of enzyme activity and malonyl-CoA binding in rat liver mitochondria

Carnitine palmitoyltransferase activity and malonyl-CoA binding capacity have been studied in Triton X-100 extracts and membrane residues of rat liver mitochondria. Rat liver mitochondria extracted twice with 0.5% Triton X-100 in a salt-free medium showed increased specific binding of [2-14C]malonyl-CoA when compared with intact mitochondria. High malonyl-CoA binding required the presence of salts and was inhibited by albumin. Further solubilization of the membrane residues in the Triton/KCl medium and subsequent hydroxylapatite chromatography gave a complete separation of carnitine palmitoyltransferase and malonyl-CoA binding. The results show that malonyl-CoA binds to mitochondrial component(s) which is different from and more difficult to extract from the mitochondrial membrane than most of the carnitine palmitoyltransferase.     

Carnitine palmitoyltransferase. Activation by palmitoyl-CoA and inactivation by malonyl-CoA

Extraction of rat liver mitochondria twice with 0.5% Triton X-100 in a salt-free medium leaves less than 10% of the carnitine palmitoyltransferase membrane bound. The remaining membrane-bound enzyme is inhibited virtually completely by 10 microM malonyl-CoA. Preincubation of the extracted membranes with palmitoyl-CoA and salts (KCI) for several minutes activates the enzyme and makes it increasingly insensitive to malonyl-CoA. Addition of malonyl-CoA to the preincubation reverses this desensitization. In albumin-containing media salts also decrease the binding of palmitoyl-CoA to albumin and stimulate carnitine palmitoyltransferase by increasing substrate availability in free solution. The reverse reaction shows accelerated desensitization by palmitoylcarnitine and resensitization by malonyl-CoA.     

Carnitine and derivatives in rat tissues

1. Free carnitine, acetylcarnitine, short-chain acylcarnitine and acid-insoluble carnitine (probably long-chain acylcarnitine) have been measured in rat tissues. 2. Starvation caused an increase in the proportion of carnitine that was acetylated in liver and kidney; at least in liver fat-feeding had the same effect, whereas a carbohydrate diet caused a very low acetylcarnitine content. 3. In heart, on the other hand, starvation did not cause an increase in the acetylcarnitine/carnitine ratio, whereas fat-feeding caused a decrease. The acetylcarnitine content of heart was diminished by alloxan-diabetes or a fatty diet, but not by re-feeding with carbohydrate. 4. Under conditions of increased fatty acid supply the acid-insoluble carnitine content was increased in heart, liver and kidney. 5. The acylation state of carnitine was capable of very rapid change. Concentrations of carnitine derivatives varied with different methods of obtaining tissue samples, and very little acid-insoluble carnitine was found in tissues of rats anaesthetized with Nembutal. In liver the acetylcarnitine (and acetyl-CoA) content decreased if freezing of tissue samples was delayed; in heart this caused an increase in acetylcarnitine. 6. Incubation of diaphragms with acetate or dl-β-hydroxybutyrate caused the acetylcarnitine content to become elevated. 7. Perfusion of hearts with fatty acids containing an even number of carbon atoms, dl-β-hydroxybutyrate or pyruvate resulted in increased contents of acetylcarnitine and acetyl-CoA. Accumulation of these acetyl compounds was prevented by the additional presence of propionate or pentanoate in the perfusion medium; this prevention was not due to extensive propionylation of CoA or carnitine. 8. Perfusion of hearts with palmitate caused a severalfold increase in the content of acid-insoluble carnitine; this increase did not occur when propionate was also present. 9. Comparison of the acetylation states of carnitine and CoA in perfused hearts suggests that the carnitine acetyltransferase reactants may remain near equilibrium despite wide variations in their steady-state concentrations. This is not the case with the citrate synthase reaction. It is suggested that the carnitine acetyltransferase system buffers the tissue content of acetyl-CoA against rapid changes.     

Retinol-binding protein messenger RNA levels in the liver and in extrahepatic tissues of the rat

A retinol-binding protein (RBP) cDNA clone was used to examine the effect of retinol status on the level of RBP mRNA in the liver, and to explore whether extrahepatic tissues contain RBP mRNA. In the first series of experiments, poly(A+) RNA was isolated from the livers of normal, retinol-depleted, and retinol-repleted rats and the levels of RBP mRNA in these samples were determined by both Northern blot and RNA Dot blot analyses. The levels of RBP mRNA in liver were similar in all three groups of rats. These findings confirm and extend previous studies which showed that retinol did not alter the in vivo rate of RBP synthesis or the translatable levels of RBP mRNA. In a second series of experiments, the RBP cDNA clone was used to survey poly (A+) RNA isolated from 12 different rat tissues for RBP mRNA by Northern blot analysis. We found that, along with the liver, many extrahepatic tissues contained RBP mRNA. Kidney contained RBP mRNA at a level of 5-10% of that of the liver, and the lungs, spleen, brain, stomach, heart, and skeletal muscle contained 1-3% of that of the liver. Translation of kidney poly (A+) RNA in rabbit reticulocyte lysates and immunoprecipitation of the translation products with anti-RBP antiserum resulted in a protein band of the same size as liver preRBP. These data suggest that RBP is synthesized in many extrahepatic tissues.It is possible that this extra-hepatically synthesized RBP may function in the recycling of retinol from these tissues back to the liver or to other target organs.     

The effect of DL-2-bromopalmitate on the utilization of palmitic acid by rat granular pneumocytes.

The effect of DL-2-bromopalmitate (BrPA), an analogue of palmitic acid (PA), on the utilization of this fatty acid by rat lungs was investigated by a combination of anatomic and biochemical methods. The experiments were performed in vitro on two types of preparations, isolated perfused lungs and lung slices. In the isolated lung preparation the substrate reached the lung via the capillaries, in lung slices via the alveolar epithelium. Electron microscope autoradiography showed that BrPA depressed uptake of PA by granular pneumocytes. Radioactivity recovered by tissue analysis and capture of CO2 established that PA oxidation and incorporation into phospholipids and triglycerides was depressed by BrPA. A close correlation was found between the reduction in radioactivity in phospholipids and the grain density over lamellar bodies. The study shows that BrPA reversibly interferes with the uptake and utilization of long chain fatty by granular pneumocytes. BrPA appears as a useful tool to study palmitate metabolism and surfactant production by the lung.     

Comparison of the activities of some peroxisomal and extraperoxisomal lipid-metabolizing enzymes in liver and extrahepatic tissues of the rat.

Peroxisomal (acyl-CoA oxidase and peroxisomal dihydroxyacetone-phosphate acyltransferase) and extraperoxisomal (mitochondrial fatty acid oxidation, extraperoxisomal dihydroxyacetone-phosphate acyltransferase, mitochondrial and microsomal glycerophosphate acyltransferases) lipid-metabolizing enzymes were measured in homogenates from rat liver and from seven extrahepatic tissues. Except for jejunal mucosa and kidney, extrahepatic tissues contained very little acyl-CoA oxidase activity. Peroxisomal dihydroxyacetone-phosphate acyltransferase, taken as the activity that was not inhibited by 5 mM-glycerol 3-phosphate, was present in all tissues examined, and its specific activity in liver and extrahepatic tissues was roughly of the same order of magnitude. Clofibrate treatment increased the activity of acyl-CoA oxidase in liver, and to a smaller extent also in kidney, but did not influence the activity of peroxisomal dihydroxyacetone-phosphate acyltransferase. Comparison of the activities of peroxisomal and extraperoxisomal lipid-metabolizing enzymes in extrahepatic tissues and in liver, an organ in which the contribution of peroxisomes to fatty acid oxidation and to glycerolipid synthesis has been estimated previously, suggests that, as in liver, peroxisomal long-chain fatty acid oxidation is of minor quantitative importance in extrahepatic tissues, but that in these tissues (micro)-peroxisomes are responsible for most of the dihydroxyacetone phosphate acylation and, consequently, for initiating ether glycerolipid synthesis.     

Synthesis of hepatic lipase in liver and extrahepatic tissues

Immunoprecipitations of hepatic lipase from pulse-labeled rat liver have demonstrated that hepatic lipase is synthesized in two distinct molecular weight forms, HL-I (Mr = 51,000) and HL-II (Mr = 53,000). Both forms are immunologically related to purified hepatic lipase, but not to lipoprotein lipase. HL-I and HL-II are also kinetically related and represent different stages of intracellular processing. Glycosidase experiments suggest that HL-I is the high mannose microsomal form of the mature, sialylated HL-II enzyme. Hepatic lipase activity was detected in liver and adrenal gland but was absent in brain, heart, kidney, testes, small intestine, lung, and spleen. The adrenal and liver lipase activities were inhibited in a similar dose-dependent manner by hepatic lipase antiserum. Immunoblot analysis of partially purified adrenal lipase showed an immunoreactive band co-migrating with HL-II at 53,000 daltons which was absent in a control blot treated with preimmune serum. Adrenal lipase and authentic hepatic lipase yielded similar peptide maps, confirming the presence of the lipase in adrenal gland. However, incorporation of L-[35S]methionine into immunoprecipitable hepatic lipase was not detected in this tissue. In addition, Northern blot analysis showed the presence of hepatic lipase mRNA in liver but not adrenal gland. The presence of hepatic lipase in adrenal gland in the absence of detectable synthesis or messenger suggests that hepatic lipase originates in liver and is transported to this extrahepatic site.     

The degradation of very low density lipoprotein by the extrahepatic tissues of the rat.

The supradiaphragmatic rat was used to investigate the metabolism by the extrahepatic tissues of endogenous plasma VLDL of d less than 1.006 g/ml. The demonstration that, at 20, 30 and 40 min after the isolation of the supradiaphragmatic rat, the VLDL lose respectively 29, 54, and 63% of their triglyceride provides evidence for the suitability of this preparation for the investigation of VLDL degradation. At all time intervals after the isolation of the supradiaphragmatic rat, VLDL triglyceride loss was accompanied by similar losses of cholesterol, protein and phospholipid, with the result that the percentage by weight composition of the residual VLDL remained unaltered. By subfractionation of the VLDL, a group of particles with an Sf range of 20--60 were isolated that, when compared with total VLDL, were enriched in their cholesterol (P less than 0.02), protein (P less than 0.001) and phospholipid (P less than 0.01) content. However, these particles represented only a small percentage of the total VLDL mass. Furthermore, their amount was not increased in the circulation of the supradiaphragmatic rat. The amount of IDL (d = 1.006--1.019 g/m) and of LDL (d = 1.019--1.063 g/ml) was increased in the supradiaphragmatic rat and a part of the total cholesterol and protein lost from the VLDL could be accounted for by the increases in these constituents in the IDL and LDL fractions. It is suggested that, although the liver probably takes up partial degradation products of VLDL in the intact animal, the extrahepatic tissues alone can metabolize VLDL to LDL of d = 1.019--1.063 g/ml. The lipoprotein particles taken up by the liver in the intact animal appear most likely to be those of Sf greater than 100.     

Inhibition of rat liver phosphofructokinase-2 by phosphoenolpyruvate and ADP

Phosphofructokinase-2 from rat liver is inhibited by phosphoenolpyruvate and ADP. Phosphoenolpyruvate reduces the maximum activity in respect to fructose-6-phosphate and ATP but does not give rise to complete inhibition of phosphofructokinase-2. ADP increases the apparent Michaelis constant of the enzyme for ATP and leaves the maximum activity in respect to ATP unchanged. The apparent Michaelis constant for fructose-6-phosphate is not influenced by ADP.     

Quantitation of mRNAs specific for the mixed-function oxidase system in rat liver and extrahepatic tissues during development

Evaluation of ontogenetic expression of the cytochrome P450PCN and cytochrome P450b gene families as well as the NADPH-cytochrome P450 oxidoreductase and epoxide hydrolase genes in Holtzmann rats showed that basal levels of mRNAs encoding these enzymes could be detected in most tissues. Distinct developmental patterns of mRNA expression are evident for these four proteins in liver and extrahepatic tissues. Levels of cytochrome P450b-like mRNA were comparable in adult lung and liver, while cytochrome P450PCN-homologous mRNA exhibited low levels in lung and approximately 100-fold higher levels in liver. Cytochrome P450PCN-homologous mRNA also reached substantial levels in adult intestine, and was also present in placenta, where it increased approximately 4-fold 24 h before birth. Epoxide hydrolase mRNA was demonstrated to be highest in liver followed by kidney, lung, and intestine but was extremely low in brain. NADPH-cytochrome P450 oxidoreductase mRNA in kidney, lung, prostate, adrenal, and intestine exhibited levels comparable to that found in liver; however, the pattern of expression for oxidoreductase mRNA was unique in that levels declined at maturity in liver, kidney, and intestine but not in lung and brain. Development of mixed-function oxidase and epoxide hydrolase activities in liver was distinct from that in other tissues in that mRNAs for all four proteins rose dramatically after parturition. Testis from immature males demonstrated low levels of all the mRNAs assayed, which ranged from 20% (oxidoreductase) to less than 1% (cytochrome P450PCN and epoxide hydrolase) of the levels found in liver.     

Inhibition in vitro of acyl-CoA dehydrogenases by 2-mercaptoacetate in rat liver mitochondria.

In rat liver hypo-osmotically treated mitochondria, 2-mercaptoacetate inhibits respiration induced by palmitoyl-CoA, octanoate or butyryl-CoA only when the reaction medium is supplemented with ATP. Under this condition, NADH-stimulated respiration is not affected. In liver mitochondrial matrix, the presence of ATP is also required to observe a 2-mercaptoacetate-induced inhibition of acyl-CoA dehydrogenases tested with palmitoyl-CoA, butyryl-CoA or isovaleryl-CoA as substrate. As the oxidation of these substrates is also inhibited by the incubation medium resulting from the reaction of 2-mercaptoacetate with acetyl-CoA synthase, with conditions under which 2-mercaptoacetate has no effect, 2-mercaptoacetyl-CoA seems to be the likely inhibitory metabolite responsible for the effects of 2-mercaptoacetate. Kinetic experiments show that the main effect of the 2-mercaptoacetate-active metabolite is to decrease the affinities of fatty acyl-CoA dehydrogenases towards palmitoyl-CoA or butyryl-CoA and of isovaleryl-CoA dehydrogenase towards isovaleryl-CoA. Addition of N-ethylmaleimide to mitochondrial matrix pre-exposed to 2-mercaptoacetate results in the immediate reversion of the inhibitions of palmitoyl-CoA and isovaleryl-CoA dehydrogenations and in a delayed reversion of butyryl-CoA dehydrogenation. These results led us to conclude that (i) the ATP-dependent conversion of 2-mercaptoacetate into an inhibitory metabolite takes place in the liver mitochondrial matrix and (ii) the three fatty acyl-CoA dehydrogenases and isovaleryl-CoA dehydrogenase are mainly competitively inhibited by this compound. Finally, the present study also suggests that the inhibitory metabolite of 2-mercaptoacetate may bind non-specifically to, or induce conformational changes at, the acyl-CoA binding sites of these dehydrogenases.     

The relationship of rat liver overt carnitine palmitoyltransferase to the mitochondrial malonyl-CoA binding entity and to the latent palmitoyltransferase.

Concanavalin A (ConA) stimulated the phosphorylation of the beta-subunit of the insulin receptor and an Mr-185,000 protein on serine and tyrosine residues in intact H-35 rat hepatoma cells. This Mr-185,000 protein whose phosphorylation was stimulated by ConA was identical to pp185, a protein reported previously to be a putative endogenous substrate for the insulin receptor tyrosine kinase in rat hepatoma cells. In Chinese hamster ovary (CHO) cells transfected with cDNA of the human insulin receptor, tyrosine-phosphorylation of pp185 was strongly enhanced by ConA compared with the controls, suggesting that the induction of tyrosine-phosphorylation of pp185 was due to stimulation of the insulin receptor kinase by ConA. Moreover, monovalent ConA only slightly induced the tyrosine-phosphorylation of pp185, which was enhanced by the addition of anti-ConA IgG, suggesting that ConA stimulated the insulin receptor kinase mainly by the receptor cross-linking or aggregation in intact cells. These data suggest that the insulin-mimetic action of ConA is related to the autophosphorylation and activation of the insulin receptor tyrosine kinase, as well as the subsequent phosphorylation of pp185 in intact cells.     

Interacting effects of L-carnitine and malonyl-CoA on rat liver carnitine palmitoyltransferase.

Malonyl-CoA significantly increased the Km for L-carnitine of overt carnitine palmitoyltransferase in liver mitochondria from fed rats. This effect was observed when the molar palmitoyl-CoA/albumin concentration ratio was low (0.125-1.0), but not when it was higher (2.0). In the absence of malonyl-CoA, the Km for L-carnitine increased with increasing palmitoyl-CoA/albumin ratios. Malonyl-CoA did not increase the Km for L-carnitine in liver mitochondria from 24h-starved rats or in heart mitochondria from fed animals. The Km for L-carnitine of the latent form of carnitine palmitoyltransferase was 3-4 times that for the overt form of the enzyme. At low ratios of palmitoyl-CoA/albumin (0.5), the concentration of malonyl-CoA causing a 50% inhibition of overt carnitine palmitoyltransferase activity was decreased by 30% when assays with liver mitochondria from fed rats were performed at 100 microM-instead of 400 microM-carnitine. Such a decrease was not observed with liver mitochondria from starved animals. L-Carnitine displaced [14C]malonyl-CoA from liver mitochondrial binding sites. D-Carnitine was without effect. L-Carnitine did not displace [14C]malonyl-CoA from heart mitochondria. It is concluded that, under appropriate conditions, malonyl-CoA may decrease the effectiveness of L-carnitine as a substrate for the enzyme and that L-carnitine may decrease the effectiveness of malonyl-CoA to regulate the enzyme.