Structural determinants of the platelet agonist activity of cotton bract condensed tannin

Previous studies on the platelet agonist activity of cotton bract condensed tannin have used tannins with apparent molecular weights greater than 10,000 Da as measured by dialysis and/or ultrafiltration. However, because tannins in cotton dust are heterogeneous in terms of molecular weight, it was important to determine if platelet agonist activity was related to polymer length. To accomplish this, aqueous extracts of cotton dust were fractionated by a series of graded Amicon ultrafiltrations and the resulting fractions were examined for tannin content and platelet agonist activity. All tannins with apparent molecular weight greater than 1000 Da were equally potent platelet agonists while those less than 1000 Da had no measurable platelet agonist activity. This polymer length dependence for activity was supported by the observation that two procyanidin dimers had only minimal platelet agonist activity. In addition, methylation of the tannin hydroxyl groups resulted in the loss of platelet agonist activity. Thus, the platelet agonist activity of tannin requires a minimum polymer length and the presence of free hydroxyl groups.