First conceptualized as a mechanism for the mitochondrial transport of long-chain fatty acids in the early 1960s, the carnitine palmitoyltransferase (CPT) system has since come to be recognized as a pivotal component of fuel homeostasis. This is by virtue of the unique sensitivity of the outer membrane CPT I to the simple molecule, malonyl-CoA. In addition, both CPT I and the inner membrane enzyme, CPT 11, have proved to be loci of inherited defects, some with disastrous consequences. Early efforts using classical approaches to characterize the CPT proteins in terms of structure/function/regulatory relationships gave rise to confusion and protracted debate. By contrast, recent application of molecular biological tools has brought major enlightenment at an exponential pace. Here we review some key developments of the last 20 years that have led to our current understanding of the physiology of the CPT system, the structure of the CPT isofornis, the chromosomal localization of their respective genes, and the identification of mutations in the human population.

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We validate the use of 1H magnetic resonance spectroscopy (MRS) to quantitatively differentiate between adipocyte and intracellular triglyceride (TG) stores by monitoring the TG methylene proton signals at 1.6 and 1.4 ppm, respectively. In two animal models of intracellular TG accumulation, intrahepatic and intramyocellular TG accumulation was confirmed histologically. Consistent with the histological changes, the methylene signal intensity at 1.4 ppm increased in both liver and muscle, whereas the signal at 1.6 ppm was unchanged. In response to induced fat accumulation, the TG concentration in liver derived from 1H MRS increased from 0 to 44.9 ± 13.2 μmol/g, and this was matched by increases measured biochemically (2.1 ± 1.1 to 46.1 ± 10.9 μmol/g). Supportive evidence that the methylene signal at 1.6 ppm in muscle is derived from investing interfascial adipose tissue was the finding that, in four subjects with generalized lipodystrophy, a disease characterized by absence of interfacial fat, no signal was detected at 1.6 ppm; however, a strong signal was seen at 1.4 ppm. An identical methylene chemical shift at 1.4 ppm was obtained in human subjects with fatty liver where the fat is located exclusively within hepatocytes. In experimental animals, there was a close correlation between hepatic TG content measured in vivo by 1H MRS and chemically by liver biopsy [ R = 0.934; P < .0001; slope 0.98, confidence interval (CI) 0.70–1.17; y-intercept 0.26, CI −0.28 to 0.70]. When applied to human calf muscle, the coefficient of variation of the technique in measuring intramyocellular TG content was 11.8% in nonobese subjects and 7.9% in obese subjects and of extramyocellular (adipocyte) fat was 22.6 and 52.5%, respectively. This study demonstrates for the first time that noninvasive in vivo 1H MRS measurement of intracellular TG, including that within myocytes, is feasible at 1.5-T field strengths and is comparable in accuracy to biochemical measurement. In addition, in mixed tissue such as muscle, the method is clearly advantageous in differentiating between TG from contaminating adipose tissue compared with intramyocellular lipids.

Hyperinsulinemia, loss of glucose-stimulated insulin secretion (GSIS), and peripheral insulin resistance coexist in non-insulin-dependent diabetes mellitus (NIDDM).Because free fatty acids (FFA) can induce these same abnormalities, we studied their role in the pathogenesis of the NIDDM of obese Zucker diabetic fatty (ZDF-drt) rats from 5 weeks of age (before the onset ofhyperglycemia) until 14 weeks. Two weeks prior to hyperglycemia, plasma FFA began to rise progressively, averaging 1.9 ± 0.06 mM at the onset of hyperglycemia (P < 0.001 vs. controls). At this time GSIS was absent and fl-cell GLUT-2 glucose transporter was decreased. The triacylglycerol content of prediabetic islets rose to 10 times that of controls and was correlated with plasma FFA (r = 0.825; P < 0.001), which, in turn, was correlated with the plasma glucose concentration (r = 0.873; P < 0.001). Reduction of hyperlipacidemia to 1.3 ± 0.07 mM by pair feeding with lean littermates reduced all (I-cell abnormalities and prevented hyperglycemia. Normal rat islets that had been cultured for 7 days in medium containing 2 mM FFA exhibited increased basal insulin secretion at 3 mM glucose, and first-phase GSIS was reduced by 68%; in prediabetic islets, first-phase GSIS was reduced by 69% by FFA. The results suggest a role for hyperlipacidemia in the pathogenesis of NIDDM; resistance to insulin-mediated antilipolysis is invoked to explain the high FFA despite hyperinsulinemia, and sensitivity of (3 cells to hyperlipacedemia is invoked to explain the FFA-induced loss of GSIS.Despite decades of intensive research the pathogenesis of non-insulin-dependent diabetes mellitus (NIDDM), a disorder that affects 2-5% of the world's population, remains obscure. Because it may precede the onset of hyperglycemia by many years, insulin resistance is widely viewed as the primary abnormality in the disease (1). In this formulation the associated hyperinsulinemia is viewed as a secondary compensation by 13 cells for the antecedent insulin insensitivity; when hyperglycemia begins, it is regarded as reflecting an inability of hypersecreting 13 cells to meet an ever-increasing insulin requirement (2). However, neither the mechanism by which 13 cells initially maintain a high enough level of insulin secretion to prevent hyperglycemia despite increasing insulin resistance nor the cause of their ultimate failure to do so has been identified. 13-Cell failure is accompanied in human and rodent NIDDM by complete loss of glucose-stimulated insulin secretion (GSIS) (3,4) and, in all rodent models thus far studied, by a parallel reduction in ,8 cells displaying GLUT-2, the high-Km facilitative glucose transporter (4-7).Long-chain fatty acids, which may be central to the development of insulin resistance in NIDDM (8, 9), can stimulate basal insulin secretion (10-13) and inhibit GSIS in isolated islets (13)(14)(15)(16)(17). This suggests a scheme that could account for the 13-cell abnormalities in pre-NIDDM and NIDDM and explain the relationship between insulin resistance and...

A B S T R A C T Studies on the oxidation of oleic and octanoic acids to ketone bodies were carried out in homogenates and in mitochondrial fractions of livers taken from fed and fasted rats. Malonyl-CoA inhibited ketogenesis from the former but not from the latter substrate. The site of inhibition appeared to be the carnitine acyltransferase I reaction. The effect was specific and easily reversible. Inhibitory concentrations were in the range of values obtained in livers from fed rats by others. It is proposed that malonylCoA functions as both precursor for fatty acid synthesis and suppressor of fatty acid oxidation. As such, it might be an important element in the carbohydrate-induced sparing of fatty acid oxidation.

Non-esterified fatty acids (NEFA) serve as an important energy source for most body tissues, particularly during periods of food deprivation, but recent evidence suggests that these same molecules subserve a much broader function in whole body fuel homeostasis by virtue of their ability to act as potent signalling entities in a variety of cellular processes. One such auxiliary role of NEFA is to heighten the responsiveness of the pancreatic beta cell to a variety of insulin secretagogues. Importantly, this fatty acid-beta cell interaction, though designed by nature for physiological purposes, can, under certain circumstances, take on a pathophysiological dimension. Some new developments surrounding this Jekyll and Hyde character of fatty acids will be reviewed briefly below.NEFA and normal beta-cell function (i) The case for glucose-fatty acid cross-talk in the control of insulin secretion It is generally agreed that in order to stimulate insulin secretion, glucose must first enter the beta cell via a glucose transporter and then be metabolized to a point beyond pyruvate in a process initiated by the high K m enzyme, glucokinase. This in turn is thought to cause an increase in the ATP:ADP ratio, closure of the cell surface K + ATP channels, cell depolarization and opening of the voltage-sensitive Ca 2 channels, leading to a rise in intracellular Ca 2+ [Ca 2+ ] i and activation of exocytosis [1]. Additional mechanisms contribute, however, to the regulation of insulin secretion in the whole animal setting [2]. One of these, referred to as the K + ATP channel-independent pathway, augments the response to a raised [Ca 2+ ] i generated through the more classical pathway. A second, referred to as the K + ATP channel-independent, Ca 2+ -independent pathway of glucose signalling, appears to involve a GTP-dependent step that is activated through the combined effects of protein kinase A (PKA) and protein kinase C (PKC).Although details of these partially overlapping signalling systems remain to be worked out, yet another element must now be brought into the discussion. This has to do with the powerful influence of glucose metabolism on the intracellular disposition of fatty acids and the potential role of this interaction in stimulus-secretion coupling. That fatty acids can considerably enhance glucose-stimulated insulin secretion (GSIS) in intact animals and humans was recognized in early studies from a number of laboratories [3±10] but since many interventions that modulate NEFA concentrations also alter glucose uptake [11], it was often felt that changes in insulin sensitivity could explain most of the fluctuations in plasma insulin concentrations. Recent studies specifically designed to monitor insulin secretion patterns following manipulation of the plasma NEFA concentration have, however, generated renewed interest in the importance of these substrates in governing beta-cell function [12, 13; see below].Efforts to elucidate how fatty acids influence betacell function have led to a series of important findi...

The requirement for carnitine and the malonyl-CoA sensitivity of carnitine palmitoyl-transferase I (EC 2.3.1.21) were measured in isolated mitochondria from eight tissues of animal or human origin using fixed concentrations of palmitoyl-CoA (50 microM) and albumin (147 microM). The Km for carnitine spanned a 20-fold range, rising from about 35 microM in adult rat and human foetal liver to 700 microM in dog heart. Intermediate values of increasing magnitude were found for rat heart, guinea pig liver and skeletal muscle of rat, dog and man. Conversely, the concentration of malonyl-CoA required for 50% suppression of enzyme activity fell from the region of 2-3 microM in human and rat liver to only 20 nM in tissues displaying the highest Km for carnitine. Thus, the requirement for carnitine and sensitivity to malonyl-CoA appeared to be inversely related. The Km of carnitine palmitoyltransferase I for palmitoyl-CoA was similar in tissues showing large differences in requirement for carnitine. Other experiments established that, in addition to liver, heart and skeletal muscle of fed rats contain significant quantities of malonyl-CoA and that in all three tissues the level falls with starvation. Although its intracellular location in heart and skeletal muscle is not known, the possibility is raised that malonyl-CoA (or a related compound) could, under certain circumstances, interact with carnitine palmitoyltransferase I in non-hepatic tissues and thereby exert control over long chain fatty acid oxidation.

We asked whether the well known starvation-induced impairment of glucose-stimulated insulin secretion (GSIS) seen in isolated rat pancreas preparations also applies in vivo. Accordingly, fed and 18-24-h-fasted rats were subjected to an intravenous glucose challenge followed by a hyperglycemic clamp protocol, during which the plasma-insulin concentration was measured. Surprisingly, the acute (5 min) insulin response was equally robust in the two groups. However, after infusion of the antilipolytic agent, nicotinic acid, to ensure low levels of plasma FFA before the glucose load, GSIS was essentially ablated in fasted rats, but unaffected in fed animals. Maintenance of a high plasma FFA concentration by coadministration of Intralipid plus heparin to nicotinic acid-treated rats (fed or fasted), or further elevation of the endogenous FFA level in nonnicotinic acidtreated fasted animals by infusion of etomoxir (to block hepatic fatty acid oxidation), resulted in supranormal GSIS. The in vivo findings were reproduced in studies with the perfused pancreas from fed and fasted rats in which GSIS was examined in the absence and presence of palmitate. The results establish that in the rat, the high circulating concentration of FFA that accompanies food deprivation is a sine qua non for efficient GSIS when a fast is terminated. They also serve to underscore the powerful interaction between glucose and fatty acids in normal ␤ cell function and raise the possibility that imbalances between the two fuels in vivo could have pathological consequences. ( J. Clin. Invest. 1996. 97:2728-2735.)