Regulatory role of adenosine deaminase complexing protein (dipeptidyl peptidase IV = CD26) on the malignancy marker adenosine deaminase: Effect of membrane cholesterol and phase-transition

Abstract

While adenosine deaminase (ADA) is an established malignancy marker and its clinical involvement in severe combined immunodeficiency (SCID) is understood, the biological significance of its binding to adenosine deaminase complexing protein (CP = DPPIV = CD26) remains enigmatic. The role of lipid-protein interactions in the modulation of ADA activity by membrane-bound CP was sought. ADA bound specifically to CP reconstituted in L-a-dimyristoylphosphatidylcholine (DMPC) or asolectin vesicles. Without CP, the binding and specific activity of ADA were negligible. In the presence of CP, the specific activity recovered to values similar to those in buffer. The addition of cholesterol increased the fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH)-labeled vesicles and resulted in a bell-shaped dependence of the specific activity of ADA on cholesterol concentration. Vesicles with small cholesterol ratios mimicked Rouse sarcoma virus-transformed chick embryo fibroblasts (CEF) in that both had reduced ADA activity and increased membrane fluidity. In contrast to continuous Arrhenius plots of ADA activity in solution, free or bound to CP, ADA bound to CP reconstituted in DMPC vesicles exhibited two breaks, around 25 and 13°C, yielding three lines with similar apparent activation energies (E a ). Increased ADA activity of ∼30% was observed at each of these discontinuities. A model in which active site accessibility is dependent on membrane fluidity led to a successful simulation of the phase transitions. This model could also account for ADA reduced activity associated with increased membrane fluidity in transformed vs. normal fibroblasts.