Trypsin modulating oostatic factor (TMOF), a decapeptide that directly inhibits the biosynthesis of trypsin- and chymotrypsin-like enzymes in epithelial cells of mosquito midgut and indirectly inhibits vitellogenesis in anautogenous females, has been sequenced by Fourier transform mass spectrometry analysis. The peptide has a primary amino acid sequence of NH2-Tyr-Asp-Pro-Ala-(Pro)6-COOH and probably exhibits left-handed helical conformation as was shown by computer stereoview simulation. The factor is metabolized very rapidly (half-life of 1.6 h) in intact mosquitoes when injected after the blood meal. Inhibition of trypsin biosynthesis was followed in ligated abdomens, which synthesize trypsin but do not metabolise TMOF. At concentrations of 3 x 10(-9) M and 6.8 x 10(-6) M, TMOF inhibited 50 and 90% of trypsin-like enzyme biosynthesis, respectively. Several analogs of varying chain lengths were synthesized and evaluated for biological activity using dose-response curves. Switching the positions of Tyr and Asp at the N-terminus reduced the activity of the hormone, indicating that the N-terminus is important for biological activity. Removal of two to five prolines at the C-terminus also reduced activity, indicating that both the N- and C-termini are important. Synthesis of trypsin-like isozyme was followed in several insect species using [1,3-3H]diisopropyl-fluorophosphate (DFP) in the presence of tosylamide-2-phenylethyl chloromethyl ketone. Marked reduction of [1,3-3H]diisopropyl-phosphoryl-trypsin-like derivatives was noted after TMOF treatment, as assessed by polyacrylamide gel electrophoresis. These results indicate that the biosynthesis of trypsin-like enzyme in mosquitoes and other insects may be regulated by sequence-related TMOFs.