Octanoate metabolism in isolated hepatocytes and mitochondria from fetal, newborn and adult rabbit

Abstract

Ketogenesis from endogenous fatty acids or from exogenous octanoate has been studied in isolated hepatocytes from fetal, 24-h-old newborn and adult rabbit. In fed adult rabbits, endogenous ketogenesis is low and increases sixfold in the presence of 2 mM octanoate. At birth, endogenous ketogenesis is low and markedly increases 24 h after birth but, in both cases, the addition of 2 m M octanoate does not increase the rates of ketone body production. Hepatocytes isolated from 24-h-old newborn or fed adult rabbits and incubated with [l-'4C]octanoate show a preferential channeling of fatty acid into oxidation (84-92% of octanoate metabolized). In contrast, esterification represents 43% of the amount of octanoate metabolized at birth. Chromatographic analysis of labelled triacylglycerols shows that 76 f 2% of labelled fatty acids are identified as octanoate and all of the radioactivity in the octanoate peak is due to the carboxyl carbon. In hepatocytes from term fetus, the low capacity for octanoate oxidation is associated with a high capacity for esterification, whatever the octanoate concentration in the medium. Octanoate activated to octanoyl-CoA in the cytosol of fetal hepatocyte is not oxidized in the mitochondria since carnitine acyltransferase I has a low activity at birth in the rabbit. This suggests that only a part of the octanoate pool is activated outside the mitochondria. Factors involved in the direct esterification of octanoate into triacylglycerols in term fetal hepatocytes are discussed.The transfer of long-chain fatty acid into the mitochondria is thought to be the limiting step in the development of hepatic long-chain fatty iicid oxidation in the adult rat during the transition from the fed to the fasted state [I, 21. In the rabbit liver, the capacity for long-chain fatty acid oxidation is low in the fetus and develops during the hours following birth [3]. Two events may explain the postnatal emergence of hepatic long-chain fatty acid oxidation: a twofold increase in the activity of carnitine palmitoyltransferase I and, particularly, a profound decrease in the sensitivity of this enzyme to inhibition by malonyl-CoA [4]. Medium-chain fatty acids represent a large part (60-70%) of fatty acids in rabbit milk triacylglycerols [5]. As medium-chain fatty acids are assumed to cross the mitochondria1 membranes independently of carnitine palmitoyltransferase I [l], it should be expected that significant rates of octanoate oxidation would be present in term fetal rabbit hepatocytes. However, data collected previously have shown that ketone body production from octanoate was very low in isolated fetal hepatocytes from rat and guinea pig [6, 7].The present study was thus performed to know whether a similar defect in ketogenesis from octanoate takes place in the liver of the term fetal rabbit and to investigate the metabolic fate of octanoate in hepatic cells during the postnatal period.Correspondence to J. P. Ptgorier, Centre de Recherche sur la Nutrition, 9, rue Jules Hetzel, F-92190 Meudon-Bellevue, FranceEnz...

Evidence that the sensitivity of carnitine palmitoyltransferase I to inhibition by malonyl-CoA is an important site of regulation of hepatic fatty acid oxidation in the fetal and newborn rabbit. Perinatal development and effects of pancreatic hormones in cultured rabbit hepatocytes

“…This delay in the emergence of fatty acid oxidation has been also observed, but to a lesser extent, with octanoate as substrate [25]. Indeed, in term fetal rabbit hepatocytes, 55 % of octanoate metabolized was recovered in oxidized products, with the remainder being directly esterified, whereas at 24 h after birth, 90 % of octanoate was oxidized [25]. It has been shown that the rates of oxidation of palmitoylcarnitine and octanoylcarnitine were similar in liver rabbit mitochondria isolated at birth or 24 h after birth, whereas the oxidation rates of palmitoyl-and octanoyl-CoA were 50 % lower in term fetal mitochondria than in those from newborns [6,25].…”

“…The emergence of LCFA oxidation occurs after a lag period of 6 h, thus providing an explanation for the low rate of ketone body production observed during the first hours following birth [18]. This delay in the emergence of fatty acid oxidation has been also observed, but to a lesser extent, with octanoate as substrate [25]. Indeed, in term fetal rabbit hepatocytes, 55 % of octanoate metabolized was recovered in oxidized products, with the remainder being directly esterified, whereas at 24 h after birth, 90 % of octanoate was oxidized [25].…”

“…Indeed, in term fetal rabbit hepatocytes, 55 % of octanoate metabolized was recovered in oxidized products, with the remainder being directly esterified, whereas at 24 h after birth, 90 % of octanoate was oxidized [25]. It has been shown that the rates of oxidation of palmitoylcarnitine and octanoylcarnitine were similar in liver rabbit mitochondria isolated at birth or 24 h after birth, whereas the oxidation rates of palmitoyl-and octanoyl-CoA were 50 % lower in term fetal mitochondria than in those from newborns [6,25]. This suggests that: (1) the capacity for mitochondrial fatty acid oxidation is not the rate-limiting factor at birth; and (2) the regulation of fatty acyl-CoA oxidation is located at the level of entry into mitochondria, i.e.…”

Transplacental induction of fatty acid oxidation in term fetal pigs by the peroxisome proliferator-activated receptor alpha agonist clofibrate

“…These data show that the capacity for β-oxidation and ketogenesis develops maximally in this species during the first 6–12 h after birth, and appears to be partly dependent on the development of fatty acid-activating enzymes [38]. Similarly, a low fatty oxidation rate with a high esterification was also observed in hepatocytes isolated from term fetal rabbits, whatever the octanoate concentration in the medium [39]. Consistent with the observations in term guinea pigs and rabbits, the fatty acid oxidation obtained in term pigs at the time of delivery by C-section was also low even when compared with the rate measured in newborn pigs born naturally [5].…”

Acetate represents a major product of heptanoate and octanoate β-oxidation in hepatocytes isolated from neonatal piglets

“…Indeed, mediumchain fatty acid (MCFA : 6-12 carbons) administration to 24-hold fasted piglets failed to increase the plasma β-hydroxybutyrate concentration above 40-60 µM [10,13,14], even though similar treatments induce marked ketonaemia in dogs, humans and rats [3][4][5]. The rate of acid-soluble product (ASP) generation from hepatocytes incubated with [1-"%C]octanoate or [1-"%C]oleate in piglets is 57 % lower [15,16] than in neonatal rabbits [17,18], and is a fraction of that observed in preparations from fasted adult rats [19].…”