Modulation of interleukin 2 high‐affinity binding by lymphocyte‐derived tetrahydrobiopterin: Pterins as potential participants in the control of interleukin 2 receptor assembly

In this report, we have examined whether (6R)‐tetrahydrobiopterin (H 4 biopterin) modulates the binding of interleukin 2 to high‐affinity sites of the cloned mouse cytotoxic T‐lymphocyte clone CTLL‐2. Scatchard plot analysis of the equilibrium binding data reveals increased affinity when the cells are exposed simultaneously to interleukin 2 and to the pterin. The K d values are statistically significantly reduced from 1.4 × 10 −11 M to 0.78 × 10 −11 M interleukin 2. The dissociation kinetics of the ligand were followed at 4°C after equilibrium binding under high‐affinity conditions (1.2 × 10 −10 M interleukin 2). In the presence of H 4 biopterin, the dissociation rate constant (k −1 ) decreases from 6.2 × 10 −3 min −1 to 3.0 × 10 −3 min −1 and the half‐time of dissociation increases from 106.8 min to 218.0 min. As a third approach interleukin 2 was bound to the surface of cells under high‐affinity conditions by incubation in the cold and the internalization kinetics upon warming were determined. Sigmodial‐shaped kinetics of endocytosis in control cells indicate that the internalization rates increase only gradually. The presence of H 4 biopterin causes an apparent immediate transition from higher‐order kinetics of a linear response so that maximum internalization rates are reached immediately upon warming. The data show that lymphocyte‐derived H 4 biopterin in vitro at concentrations ranging from 2–8 × 10 −7 M modulates interleukin 2 high‐affinity binding and that H 4 biopterin potentially participates in the control of interleukin 2 receptor assembly.