BINDING-SITE POLARITY AND LIGAND AFFINITY OF HOMOLOGOUS FATTY-ACID-BINDING PROTEINS FROM ANIMALS WITH DIFFERENT BODY TEMPERATURES

Binding affinity and binding-pocket polarity is determined for intrace llular fatty acid-binding protein (FABP) from aerobic muscle of Chaeno cephalus aceratus, the Antarctic icefish, and from rat heart. FABPs bi nd fatty acids via weak-bond forces (both ionic and hydrophobic), and these bond forces are temperature sensitive, yet FABPs are present in animals whose body temperatures range over nearly 40 degrees C. To inv estigate FABP's sensitivity to body temperature, fatty acid binding af finity (K-d) was determined for both rat heart-FABP and icefish heart- FABP at two physiological temperatures (0 degrees C or 37 degrees C). Saturated and unsaturated fatty acids (16:0 and 16:1), delivered in mo del membranes (liposomes) whose composition is typical of either Antar ctic fish (16:0/22:6 phosphatidylcholine) or mammals (bovine-heart pho sphatidylcholine) were examined. Incubation at 0 degrees C or 37 degre es C does not significantly affect K-d for rat heart FABP, regardless of liposome composition or fatty acid ligand (K-d = 0.686 +/- 0.127 - 1.129 +/- 0.356 mu M at 0 degrees C, 0.775 +/- 0.307 - 1.605 +/- 0.427 mu M at 37 degrees C). Incubation temperature significantly affects i cefish FABP's affinity for 16:1 (0.626 +/- 0.093 mu M at 37 degrees C vs. 1.896 +/- 0.343 mu M at 0 degrees C for fatty acid presented in An tarctic fish liposomes; 0.331 +/- 0.101 mu M at 37 degrees C vs. 0.949 +/- 0.121 mu M at 0 degrees C for bovine heart liposomes) but not 16: 0. K-d is not significantly different between FABPs under any set of c onditions (with one exception: K-d is significantly lower in rat FABP vs. icefish FABP for 16:0 at 0 degrees C for fatty acids delivered in bovine heart liposomes). Although K-d values are largely equivalent be tween the two FABPs, relative contributions from ionic vs. hydrophobic weak-bond forces are different between the two animals. Rat heart FAB P has a binding pocket that is significantly more nonpolar than that o f icefish FABP (as measured by quantum yield of the bound fluorescent fatty-acid analogue (PA-DPH); Q = 0.067 +/- 0.008 vs. 0.034 +/- 0.005 at 0 degrees C, 0.030 +/- 0.003 vs. 0.019 +/- 0.002 at 37 degrees C). This suggests that rat-heart FABP realizes a micromolar Kd with a grea ter reliance upon hydrophobic interactions than does icefish FABP.