Interaction of α₂-macroglobulin with L-asparaginase

Summary

Obvious protection of the catalytic activity of Esch. coli L-asparaginase by α₂-macroglobulin (α₂M) was observed under conditions otherwise propitious to the dissociation of the tetrameric molecule into inactive subunits, i.e. very diluted enzyme solutions or the presence of either SDS or urea. The degree of protection depended on enzyme and α₂M concentrations respectively, and on the preincubation time of the α₂M-enzyme mixture prior to substrate addition. The formation of a catalytically active complex between α₂M and L-asparaginase was confirmed by gel filtration on a Sephadex-G column and by polyacrylamide gel electrophoresis. The fact that the migration distance of the active complex corresponded to the migration of α₂M and the absence in that case of a migration band corresponding to the intact molecule suggest that complexing of the enzyme with α₂M prevented its dissociation into subunits and thus its inactivation. Addition of α₂M to the already dissociated enzyme molecule did not restore its catalytic activity.

Alpha₂-macroglobulin was shown to have an inhibiting effect on the proteolytic activity of almost all proteases and no effect on their esterolytic activity. Furthermore, it prevents the inhibition of esterolytic activity by some natural compounds^1–5. The effect of α₂M on other types of catalytic activity has not been investigated enough to afford a generalization of the possible role of this macroglobulin in the control of enzyme activity in the body.

This paper reports the results of an in vitro study of the effect of α₂M on the catalytic activity of an important amidase, i.e. L-asparaginase (L-asparagine amidohydrolase 3.5.1.1), which in recent years has been used in the treatment of acute lymphocytic leukemia in children^6,7.