A90720A, a Serine Protease Inhibitor Isolated from a Terrestrial Blue-Green Alga Microchaete Loktakensis

The isolation and structure elucidation of A90720A,1, a cyclic depsipeptide from the terrestrial blue-green alga Microchaete loktakensis is described.1 was isolated on the basis of its thrombin inhibitory activity and its structure was determined using spectral methods and amino acid analysis. The absolute configurations of the constituent amino acids were found to be as follows: the nucleus containsL-threonine,L-arginine,L-3-amino-6-hydroxy-2-piperidone (Ahp, i.e., glutamic acid-γ-car☐aldehyde),L-leucine,L-N-methyl-tyrosine andL-valine. The side chain off theL-threonine amino group containsD-leucine, followed by a 3-O-sulfatedR-glycerate moiety.