Gluconeogenesis from serine in rabbit hepatocytes

L-Serine alone is not gluconeogenic in isolated rabbit hepatocytes, whereas in rat liver this amino acid has been reported to yield as much glucose as does L-lactate itself. The current study has been an investigation into the explanation of the difference between the two species. Hepatocytes were isolated from 48-h-starved, 750- to 1000-g male rabbits, and the viability of each preparation was judged by ATP levels (2.4 +/- 0.2 mumol/g wet wt) at the beginning and end of the incubation as well as gluconeogenesis from 10 mM L-lactate (0.83 +/- 0.08 mumol/min/g wet wt). L-Serine alone produced virtually no glucose or pyruvate accumulation above baseline. Hydroxypyruvate, however, did appear in the incubation mixture. When L-serine and pyruvate were combined to test the functional activity of L-serine:pyruvate aminotransferase (EC 2.6.1.51), however, gluconeogenesis remained at the rate produced by pyruvate alone (0.61 +/- 0.04 mumol/min/g wet wt). On the other hand, the combination of L-serine and L-lactate produced rates of glucose accumulation 35% above that of L-lactate alone. The combination of L-lactate plus hydroxypyruvate produced nearly maximal rates (1.39 +/- 0.08 mumol/min/g wet wt), approaching those achieved by a physiologic ratio (10:1) of L-lactate and pyruvate. Hydroxypyruvate itself was only moderately gluconeogenic (0.44 +/- 0.04 mumol/min/g wet wt). That a reduction of the cytoplasmic free [NAD+]/[NADH] ratio by L-lactate was not its only contribution to L-serine utilization was suggested by the fact that ethanol completely eliminated gluconeogenesis from virtually all precursors (or combinations) tested, with the exception of hydroxypyruvate. It has been concluded from the data that, probably in contrast to the rat, the major pathway for the entrance of L-serine into gluconeogenesis in rabbit hepatocytes is through the pathway initiated by L-serine: pyruvate aminotransferase and that L-lactate is an important participant (i) by generating cytoplasmic reducing equivalents (NADH), (ii) by supplying pyruvate for the transaminating reaction itself, and, perhaps, (iii) by preventing hydroxypyruvate from being reduced by L-lactate dehydrogenase (EC 1.1.1.27) to L-glycerate.