Peptide nature of two mosquito natriuretic factors

High-pressure liquid chromatography (HPLC) of saline extracts of Aedes aegypti heads yields three fractions (from a total of 108) that affect transepithelial voltage and/or fluid secretion in isolated Aedes Malpighian tubules. In this study we investigated the physical and chemical nature of the active materials in these fractions. Gel-filtration chromatography revealed that the molecular weights of the three fractions were between 1,900 and 2,700. To test their thermostability the fractions were repeatedly frozen and thawed over a period of 110 days without loss of biological activity. Boiling at 100 degrees C for 5 min failed to significantly reduce their biological effects in isolated Malpighian tubules. In contrast, treatment with the proteolytic enzyme mixture, pronase, destroyed activity in all three. Fraction I no longer depolarized the transepithelial voltage of in vitro perfused Malpighian tubules, and fractions II and III completely lost their ability to stimulate fluid secretion and to affect transepithelial voltage. We conclude that our HPLC isolation yields a heterogeneous group of three polar low-molecular weight peptides. Expression of their biological activities in Malpighian tubules depends on intact peptide bonds.