Effects of temperature and sodium on myocardial and hepatocellular fatty acid uptake

Fatty acid influx into human myocardium was studied in 15 patients during the cooling phase of cardiopulmonary bypass at myocardial temperatures of 37 degrees to 25 degrees C. The fitting of the data to a functional relation, developed in this study, revealed fatty acid influx to be a temperature-dependent saturable process corresponding to a Michaelis-Menten constant (Km) at 37 degrees C of 0.26 +/- 0.084 mumol/g, a maximal fatty acid influx velocity (Vmax) at 37 degrees C of 0.28 +/- 0.045 mumol/g per minute, activation energy for fatty acid binding to the putative carrier (E) of 23.8 +/- 5.6 kcal/mol, and a free energy for conformational change of the carrier (U) of 10.9 +/- 8.0 kcal/mol. In short-term cultured hepatocytes, Km increased in the absence of Na+ from 171 +/- 48 to 301 +/- 71 nmol/L, and Vmax of [3H]oleate decreased from 1063 +/- 69 to 847 +/- 68 pmol/min per milligram protein. The fitting of these data to a functional relation revealed a transmembrane potential-dependent component of parameters E and U to be -0.479 and -0.374 kcal/mol, respectively. It is proposed that for fatty acid influx a protonated fatty acid form is preferred that consists of a Na+ complex with the mesomeric form of nondissociated fatty acid from which Na+ and H+ are released during collision with the carrier.