Calpain is a Ca2 +-dependent cysteine protease which is found in the microsomal and cytosolic compartments of most mammalianneurons and other cells1}. Calpain hydrolyses peptide bonds of proteins in the cytosol2'3). A large amount of research effort has been focused on this enzyme, since it potentially plays a central role in several physiological events, such as turnover of myofibrillar proteins, protein kinase C activation, cytoskeleton and cell membrane organization2*, neuropeptide

Methylpendolmycin, a new indole alkaloid with an N-methylisoleucine moiety incorporated in the nine-membered indolactam ring, has been isolated from an actinomycete culture of Nocardiopsis. Methylpendolmycin exhibited inhibition of phorbol ester binding to protein kinase C. Its structure was assigned on the basis of spectroscopic data.

Twonew peptides, a diketopiperazine of Af-methyltyrosine (1) and a tetrapeptide containing N-methyltyrosine (2), were isolated from an actinomycete strain Streptomyces griseus. These compoundsinhibit the enzyme calpain in the micromolar range and were characterized on the basis of spectroscopic analysis, amino acid analysis and sequencing. The structure of the tetrapeptide A^-methyltyrosyl-TV-methyltyrosyl-leucyl-alanine (2), was also confirmed by total synthesis.

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