Fatty Acid Synthesis During Fasting and Bovine Ketosis

A study was made of the changes which occur in the fatty acid composition of milk fat during fasting and clinical ketosis.Samples were taken from nornml Jersey, Holstein, and Guernsey cows and from the same cows following a fiveday fast or during an acute attack of ketosis.The low molecular weight fatty acids fell on the average 36 and 64 tool per cent from normal milk fat values in ketotic and fasted cows, respectively.A compensatory increase was noted in the long-chain fatty acids, palmitoleic, stearie, and especially oleic.Further, the per cent decrease in the individual low molecular weight fatty acids appeared to be inversely proportional to their concentration in normal milk fat.This observation is not consistent with the hypothesis that a specific enzymatic defect inhibits fatty acid formation from acetate during fasting or clinical ketosis.Similarly, it seems unlikely that the diminished lipogenesis from acetate results from the preferential utilization of plasma lipid fatty acids or a shift in the normal pathway of acetate metabolism ,i.e., from lipogenesis to gluconeogenesis.It was concluded that the cause of impaired acetate utilization during ketosis will remain obscure until the major pathways of fatty acid synthesis are more fully understood.The milk fat of ruminants is unique in its relatively high concentration of low molecular weight fatty acids.These acids range in carbon chain length from C-4 through C-14, and are thought to originate in mammary tissue via synthesis from acetate (15) and beta-hydroxybutyrate (8).Recently, several workers have reported that dairy cows suffering from the metabolic disease, ketosis~ incorporate less C ~4labeled acetate into milk fat than do normal cows (7, 12).Similar results have also been obtained with cows made ketotic through prolonged fasting (17).This observation led to the hypothesis that a lipogenic defect impairs acetate utilization for fat synthesis during bovine ketosis (6, 20).The defect is thought to block the addition of C-2 units somewhere in the fatty acid synthesizing sequence of enzymatic reactions that occur in mammary tissue (20) and may involve a shm~tage o£ the reduced coenzyme, triphosphopyridine nucleotide, TPN-H (21).Since acetate is generally acknowledged to be the chief precursor of the short-chain fatty acids, a biochemical aberration at this level should lead to significant changes Received for publication :May 27, 1963. 1 This work was supported in part by the U.S. Atomic Energy Commission and by funds from the California Dairy