A new family of small (5 kDa) protein inhibitors of insect alpha-amylases from seeds or sorghum (Sorghum bicolar (L) Moench) have sequence homologies with wheat gamma-purothionins.

Three isoinhibitors of locust and cockroach gut alpha-amylases were purified from seeds of sorghum by saline extraction, precipitation with ammonium sulphate, affinity chromatography on Red-Sepharose and preparative RP-HPLC on Vydac C18. The complete primary structures were determined by automatic degradation of the intact reduced and S-alkylated proteins, and by manual DABITC/PITC microsequencing of peptides obtained from enzyme digests. The inhibitors consist of 47 (SI alpha-1) or 48 (SI alpha-2, ST alpha-3) amino acids, and are the smallest plant inhibitors of alpha-amylase currently known. The sequences of the three isoinhibitors exhibit between 38% and 87% identity among themselves and also have homology (32-81%) with the gamma-purothionins recently isolated from wheat endosperm.