Enzymatic Synthesis of Dehydro Cyclo(His–Phe)s, Analogs of the Potent Cell Cycle Inhibitor, Dehydrophenylahistin, and Their Inhibitory Activities toward Cell Division

Cyclo(His-Phe) was effectively converted to its dehydro derivatives by the enzyme of Streptomyces albulus KO-23, an albonoursin-producing actinomycete. Two types of dehydro derivatives were isolated from the reaction mixture and identified as cyclo(DeltaHis-DeltaPhe) and cyclo(His-DeltaPhe). This is the first report on cyclo(His-DeltaPhe) and the enzymatic preparation of both compounds. Cyclo(DeltaHis-DeltaPhe), a tetradehydro cyclic dipeptide, exhibited a minimum inhibitory concentration of 0.78 mumol/ml inhibitory activity toward the first cleavage of sea urchin embryos, in contrast to cyclo(His-DeltaPhe) that had no activity. The finding that the isoprenylated derivative of cyclo(DeltaHis-DeltaPhe), dehydrophyenylahistin, had 2,000 times higher activity than cyclo(DeltaHis-DeltaPhe) indicates that an isoprenyl group attached to an imidazole ring of the compound was essential for the inhibitory activity.