FATTY-ACID SYNTHESIS FROM GLUTAMATE IN THE ADIPOSE-TISSUE OF NORMAL SUBJECTS AND OBESE PATIENTS - AN ENZYME STUDY

In the adipose tissue, besides fatty acid synthesis (FA-S) from glucos e, which includes several mitochondrial steps, FA-S from glutamate has been demonstrated. FA-S from glutamate takes place in the cytosol thr ough the backward pathway of Krebs cycle (BPKC) and is due to the sequ ential action of (1) alanine aminotransferase (ALT, EC 2.6.1.2), which in presence of pyruvate converts glutamate to oxoglutarate; (2) isoci trate dehydrogenase (NADP) (ICDH, EC 1.1.1.42), which converts oxoglut arate to isocitrate; (3) aconitate hydratase (AGO, EC 4.2.1.3), which transforms isocitrate to citrate; and (4) ATP citrate-lyase (ATP-CL, E C 4.1.3.8), which splits citrate to yield the acetyl-CoA needed for FA -S. We studied the enzymes involved in BPKC in homogenates of human ad ipose tissue. In normal subjects, the cytosolic activity (mu mol/min/g protein) was: ALT = 10.3 +/- 1.1, ICDH = 29.5 +/- 2.8, ACO = 2.05 +/- 0.23, and ATP-CL = 1.2 +/- 0.2. Mitochondria contained less or no act ivity, values being 20, 9, 11, and 0% of total for ALT, ICDH, AGO, and ATP-CL, respectively. BPKC enzymes are more active than the enzymes l imiting FA-S from glucose, i.e., phosphofructokinase (EC 2.7.1.11), py ruvate carboxylase (EC 6.4.1.1), and pyruvate dehydrogenase (EC 1.2.4. 1). In the obese patients, cytosolic ALT and ATP-CL were increased (12 .9 +/- 0.7, P < 0.05, and 2.28 +/- 0.27, P < 0.01, respectively) compa red to normal, while ICDH was not changed (AGO could not be studied). Similar changes were obtained by expressing enzyme activity per fat ce ll number. Besides the increased enzyme activity, the BPKC may be more active in the obese patients because of hyperinsulinemia, as insulin and glucose stimulate BPKC, perhaps by increasing the supply of pyruva te through stimulation of glycolysis. Thus BPKC may play a significant role in FA-S in human adipose tissue. (C) 1995 Academic Press, Inc.