The acidic groups of the neocarzinostatin protein play an important role in its biological activity

F Schonlau, W Kohnlein and MC Garnett

Institut fur Strahlenbiologie, Universitat Munster, Germany.

Despite the extensive literature on the mechanism of action of the anticancer agent neocarzinostatin (NCS), the role of the protein moiety is still not clear. The model involving endocytosis of intact holo-NCS has been dismissed in favor of a theory proposing entry of free dissociated chromophore. However, the fact that the NCS protein has a certain affinity for cell membranes cannot be disregarded. In the present work the protein moiety has been modified by transformation of the carboxyl groups into isopropylamide groups. This modification resulted in a broad shift of the protein pI towards higher values, accompanied by a marked reduction of toxicity in vitro and reduced binding of protein to cells. Coincubation of modified NCS with inactive native apo-NCS led to restoration of the biological activity of native holo-NCS. It has been shown chromatographically that the modified NCS is capable of transferring active chromophore to native apo-NCS. The inactive form (apoprotein) of the charge-modified NCS, however, is capable of inhibiting the toxicity of the active form, as has been described for the corresponding native pair. Binding experiments with tritium-labeled proteins revealed that the modified protein has diminished affinity for membranes, in comparison with native NCS.

Volume 45, Issue 6, pp. 1268-1272, 06/01/1994
Copyright © 1994 by American Society for Pharmacology and Experimental Therapeutics