Biochemical and biophysical analysis of the intracellular lipid binding proteins of adipocytes

Adipocytes express two lipid-binding proteins; the major one termed the adi pocyte lipid-binding protein or aP2 (ALBP/aP2) and a minor one referred to as the keratinocyte lipid-binding protein (KLBP). In order to evaluate the potential physiological roles for these proteins, their biochemical and bio physical properties have been analyzed and compared. ALBP/aP2 and KLBP exhi bit similar binding affinities for most long-chain fatty acids; however, AL BP/aP2 exhibits a two to three-fold increased affinity for myristic, palmit ic, oleic and linoleic acids, the predominant fatty acids of adipocytes. As measured by guanidinium hydrochloride denaturation, the stability of ALBP/ aP2 is nearly 3 kcal/mol greater than that of KLBP. While the pI of ALBP/aP 2 was determined to be 9.0, that of KLBP is 6.5 suggesting differing net ch arges at physiological pH. Analysis of surface electrostatic properties of ALBP/aP2 and KLBP revealed similar charge polarity, although differences in the detailed charge distribution exist between the proteins. The distribut ion of hydrophobic patches was also different between the proteins,ALBP/aP2 has only scattered hydrophobic surfaces while KLBP has a large hydrophobic patch near the ligand portal into the binding cavity. In sum, these result s point out that despite the striking similarity between ALBP/aP2 and KLBP in tertiary structure, significant differences in ligand binding and surfac e properties exist between the two proteins. Hence, while it is tempting to speculate that ALBP/aP2 and KLBP are metabolically interchangeable, carefu l analysis suggests that the two proteins are quite distinct and likely to play unique metabolic roles.