Identification of rat alpha(1) macroglobulin as an inhibitor of rat Gal beta 1-4GlcNAc alpha 2-6 sialyltransferase

Rat serum was found to contain an inhibitor of pure alpha 2-6 sialyltransferase (EC 2.4.99.1). The inhibitor was a high Mr protein isolated by molecular filtration on Sephadex G100, followed by anion exchange chromatography on Sephadex DEAE A25, then separation on Sepharose CL 4B, and finally bg isoelectric focusing, Electrophoretic separation and subsequent N-terminal sequence analysis of the active inhibitor preparation showed the presence of two protein components, identified as rat C-reactive protein, and rat alpha(1) macroglobulin. Pure rat CRP did not inhibit alpha 2-6 sialyltransferase. Treatment of the inhibitor preparations with monospecific antibodies against rat alpha(1) macroglobulin blocked inhibitory activity in a dose-dependent manner. The results present strong evidence that alpha(1) macroglobulin is capable of acting as an inhibitor of alpha 2-6 sialyltransferase. No inhibition of galactosyltransferase (EC 2.4.1.38) could be detected, indicating that the interaction with alpha(1) macroglobulin may have specificity among the glycosyltransferases. The entrapment of alpha 2-6 sialyltransferase by alpha(1) macroglobulin presented here occurs in vitro and will require further in vivo investigations to determine the precise physiological significance, Independent of the physiologic significance is the finding that this interaction occurs in vitro, which, pending elucidation of the precise mechanism and specificity, mag provide a new tool for investigations into the functional significance of sialylation, and potentially for use or design of new inhibitors of therapeutic value in physiologic conditions involving altered levels of sialylation.