The binding and processing of mannose-bovine serum albumin derivatives by rabbit alveolar macrophages. Effect of the sugar density.

Mammalian alveolar macrophages are known to bind, internalize, and degrade glycoconjugates containing terminal D-mannosyl residues such as bovine serum albumin modified with 2-imino-2-methoxyethyl 1-thio-alpha-D-mannopyranoside (Mann-AI-BSA) (Lee, Y. C., Stowell, C.P., and Krantz, M. J. (1976) Biochemistry 15, 3956-3963). In this report, the binding (2 degrees C) and initial uptake (37 degrees C) of Mann-AI-BSA (n = 5, 13, 24, and 43) by rabbit alveolar macrophages were examined. Man43-AI-BSA had about 400 times higher affinity (Kd = 2.0 nM) for the macrophage than the Man5-AI-BSA ligand (Kd = 820 nM) at 2 degrees C. Kinetic analysis of 125I-Man43-AI-BSA binding to macrophages at 2 degrees C yielded an association rate constant of 1.2 X 10(6) M-1 min-1 and a dissociation rate constant of 5.9 X 10(-3) min-1 (t1/2 = 117 min). The association rate constant at 37 degrees C was 2 orders of magnitude greater than at 2 degrees C. When the endocytotic process at 37 degrees C was analyzed by Michaelis-Menten-type kinetics, Man5-AI-BSA had a K uptake 20 times higher than that of Man43-AI-BSA (328 and 18 nM, respectively). The maximal uptake velocities at 37 degrees C were, however, very similar for all four Man-AI-BSA derivatives (62,600-83,000 molecules/cell/min). The rate constants for internalization and hydrolysis for 125I-Man43-AI-BSA and 125I-Man13-AI-BSA were determined using a steady state approach. The internalization rate constant for 125I-Man43-AI-BSA, 1.23 (+/- 0.20) min-1, was similar to that constant obtained for 125I-Man13-AI-BSA, 1.80 (+/- 0.67) min-1. The hydrolysis rate constants for the two ligands were also close, 7.4 (+/- 0.2) X 10(-2) and 9.2 (+/- 0.5) X 10(-2) min-1, for 125I-Man43-AI-BSA and 125I-Man13-AI-BSA, respectively.