[Leu1]tuftsin was reported to have greater phagocytosis-stimulating activity than tuftsin (Thr-Lys-Pro-Arg). 2. However, a study on inactivation of tuftsin by polymorphonuclear leukocytes (PMNs) demonstrated that leucine aminopeptidase, an ecto-enzyme, located on PMN surface was responsible for this mechanism. 3. Since leucine aminopeptidase is known to cleave Leu more easily than Thr at the N-terminal position of peptides, this suggested to us that [Leu1]tuftsin might then be inactivated by PMNs more easily than tuftsin, and thus this analog might be less active than tuftsin. 4. In addition, many tuftsin preparations used in earlier studies were not fully active, as high-performance liquid chromatography was not available to separate out many contaminating diastereomers. 5. In view of this, we have synthesized and purified [Leu1]tuftsin and compared its phagocytosis-stimulating activity with tuftsin. 6. Our results indicate that [Leu1]tuftsin is not as active as tuftsin in stimulating phagocytosis.
