Pii: S0041-0101(00)00224-5

A previously undescribed cyclic heptapeptide hepatotoxin was isolated from a cyanobacteria waterbloom collected in Pakowki Lake, Alberta, Canada (49820 0N and 110855 0W). The compound was characterized by amino acid analysis, ESIMS/CID/MS, H and C NMR, and UV spectroscopy. Structure of the new microcystin was assigned as [d-Leu]microcystin-LR (1). The amino acid composition is the same as microcystin-LR (2) except for d-Leu and l-Leu in 1 instead of d-Ala and l-Leu in 2. This is the ®rst microcystin identi®ed, among the 64 known microcystins, that has both a dand l-Leu amino acid. Toxicity as measured by the protein phosphatase inhibition activity of 1 is similar to microcystin-LR. The presence of microcystins in waterblooms from this lake is discussed in relation to the almost yearly bird mortalities that have occurred there since 1995. q 2001 Elsevier Science Ltd. All rights reserved.