ARE CELL REDOX OR LACTATE-DEHYDROGENASE KINETICS RESPONSIBLE FOR THE ABSENCE OF GLUCONEOGENESIS FROM LACTATE IN SEA RAVEN, HEPATOCYTES

Previous studies have reported very low rates of gluconeogenesis from lactate in sea raven (Hemitripterus americanus) hepatocytes compared t o other teleosts studied. This study examines whether hepatic cell red ox or lactate dehydrogenase (LDH) characteristics may explain this obs ervation. Sea raven hepatic optimal LDH activities (pyruvate reductase direction) were more than 40 times less compared with rainbow trout l iver values (40 vs 1914 mumol.min-1.g-1 protein). The Km(lactate) was 9.24 and 0.86 mM for sea raven and trout hepatic LDH, but the Km(pyruv ate) was similar between the two species (0.11 and 0.21 mM, respective ly). These results suggested that sea raven liver LDH did not favour l actate use and was more indicative of the mammalian M-isozyme. Gel ele ctrophoresis showed a predominant intermediate isozyme, with a small a mount of the M-type LDH. Phosphoenolpyruvate carboxykinase (PEPCK) was localized to the mitochondrial compartment, while there was no appare nt mitochondrial glutamate-oxaloacetate transaminase (GOT) activity. N o in vitro lactate flux to glucose was found in untreated, 10 mM ethan ol-treated, or 3 mM NH4Cl-treated sea raven hepatocytes, although CO2 production from lactate was decreased by ethanol and increased by NH4C l. These results provide evidence that cell redox does not limit gluco neogenesis from lactate, while low activities and the kinetic characte ristics of LDH may partially explain the low lactate gluconeogenesis r eported in sea raven hepatocytes.