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Fatty Acid Synthesis in Sheep Mammary Tissue¹

Department of Dairy Science, University of Illinois, Urbana 61801

ABSTRACT

Biochemical pathways of fatty acid synthesis in sheep mammary tissue were investigated. Ewes were sacrificed (3 to 4 wk postpartum) for mammary tissue for tissue slice incubations and enzyme assays. Acetate was readily incorporated into fatty acids by sheep mammary slices. However, lipogenesis from glucose, glycerol, and pyruvate was limited, indicating inability to utilize acetyl coenzyme A generated in the mitochondria for fatty acid synthesis. Coinciding with lack of mitochondrial acetyl coenzyme A utilization for fatty acid synthesis was an absence of two citrate-cleavage pathway enzymes (ATP-citrate lyase and NADP-malate dehydrogenase). Sources of reducing equivalents necessary to support lipogenesis were also examined. Comparison of acetate incorporation into fatty acids in the presence and absence of glucose indicated that approximately one-half of the reducing equivalents can originate from pathways not involving the pentose phosphate cycle. Results were consistent with these reducing equivalents being produced via NADP-isocitrate dehydrogenase. In sheep mammary tissue, activity of NADP-isocitrate dehydrogenase was 6 to 12-fold greater than glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase activities. Presumably because of the premium on glucose and the availability of acetate, sheep mammary tissue has adapted to spare glucose in two ways: first, to utilize acetate but not glucose as a carbon source for fatty acid synthesis and second, to generate a portion of the reducing equivalents from acetate via the isocitrate pathway.

¹ Supported in part by Illinois Agricultural Experiment Station (35-337).

² NIH predoctoral trainee; present address: Department of Dairy Science, Michigan State University, East Lansing 48823.