The metabolic fate of the products of citrate cleavage. Adenosine triphosphate citrate lyase and nicotinamide–adenine dinucleotide phosphate-linked malate dehydrogenase in foetal and adult liver from ruminants and non-ruminants

1. Foetal rat liver slices incorporate the C-3 of aspartate and C-2 of glutamate into fatty acids at rates equal to those observed with adult rat liver slices. Incorporation of either of these labelled carbon atoms into fatty acids would require a functioning citrate-cleavage pathway which consists of the enzymes ATP-citrate lyase, NAD-malate dehydrogenase and NADP-malate dehydrogenase. However, NADP-malate dehydrogenase is present in foetal rat liver at only 5% of the activity detectable in adult rat liver. 2. From these findings and the effect of cofactors on the formation of (14)CO(2) from [1,5-(14)C(2)]citrate in liver supernatant fractions (100000g), it is suggested that NADP-malate dehydrogenase limits the citrate-cleavage sequence. 3. Measurement of the citrate-cleavage pathway by incorporation studies with [3-(14)C]aspartate and [U-(14)C]glucose and by determining the activities of ATP-citrate lyase and NADP-malate dehydrogenase have shown that this sequence of reactions is present in the liver of the bovine foetus but not in the adult. However, C-2 of glutamate is not incorporated into fatty acids or non-saponifiable lipid by bovine liver slices. This finding as well as those presented above for the adult and foetal rat liver are interpreted on the basis of a competition between phosphoenolpyruvate carboxykinase and NAD-malate dehydrogenase for oxaloacetate produced by the cleavage of citrate in the cytosol.