Pathways of fatty acid synthesis and reducing equivalent generation in mammary gland of rat, sow, and cow

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Glucose was incorporated into fatty acids to a much greater extent by mammary tissue slices of the rat than by cow tissue slices. It was demonstrated that this is the consequence of a lack of ATP-citrate lyase and NADP-malate dehydrogenase in cow mammary cytosol. It was further shown that β-hydroxybutyrate dehydrogenase is located almost exclusively in the mitochondria of cow mammary tissue. Thus acetyl-CoA arising from dissimulation of β-hydroxybutyrate within the mitochondria is not available for fatty acid synthesis in the cytosol. This observation is consistent with the findings that β-hydroxybutyrate's primary contribution to fatty acid synthesis is as a four-carbon unit at the methyl-terminal end of the carbon chain. Mammary tissue experiments demonstrated that in contrast to rat and sow, the tissue from the cow incorporated substantial quantities of acetate into fatty acids in the absence of glucose. This finding coupled with a lack of NADP-malate dehydrogenase and a reduced glucose-6-phosphate dehydrogenase activity suggests that the tissue of the cow has alternate sources of NADPH. The cytosol activity of NADP-isocitrate dehydrogenase was shown to be 20-fold greater in cow than in rat mammary tissue. A pathway for the generation of NADPH in cow mammary tissue involving this enzyme is proposed.