Comparative measurement of the molecular weight of an antineoplastic glucan from BCG vaccine

Bacillus Calmette-Guerin (BCG) vaccine, developed originally for the prophy laxis of tuberculosis, is a potent immunostimulant used to treat superficia l bladder carcinoma in man. The aim of this study was to compare the molecu lar weight and self-association properties of an antineoplastic glucan (PS1 A1) extracted from BCG vaccine as determined by different techniques includ ing diffusion, light-scattering and chromatographic methods. In the diffusion experiments, a semi-empirical relationship was derived bet ween the effective diffusion coefficients, D-p, and the weight-average mole cular weights, M-W, of several dextrans used as standards, according to the equation D-p=2.233 x 10(-6) x M-W (-0.66). On the basis of this relationsh ip, the molecular weight of PS1A1 was found to be 57.4 kDa, although, unexp ectedly, membrane association was high, most probably because of molecular branching. In the light-scattering experiment it was observed that, unlike dextran, PS1A1 undergoes concentration-dependent multimerization in water. However, the molecular weight of PS1A1 in 0.1M sodium chloride ranged from 60 to 68 kDa, with a mean of 65 kDa, over the same concentration range. Thi s value was in agreement with the molecular weight determined for PS1A1 by gel-filtration chromatography in previous studies, suggesting that 65 kDa r epresents the approximate monomeric size of the unassociated molecule. Thus, it was evident that the aggregation was suppressed by electrolyte. El emental analysis by X-ray fluorescence showed that PS1A1 contained carbon, oxygen, hydrogen and phosphorus, indicating that hitherto unobserved ionize d phosphate groups might promote electrostatic interactions.