De novo synthesis of methionine in normal and Brugia-infected Aedes aegypti.

Crude extracts of normal, adult Aedes aegypti were able to form methionine from homocysteine in the presence of 5-methyltetrahydrofolate (MeFH4) but not betaine. The requirements for the reaction, including a need for vitamin B12, S-adenosylmethionine (SAM), and a reducing system, indicated that it was catalyzed by MeFH4:homocysteine transmethylase (methionine synthetase). The general properties of A. aegypti methionine synthetase were found to be similar to those of the analogous enzyme from bacterial and mammalian sources, except that its apparent affinity for SAM was significantly lower. Extracts of normal, adult A. aegypti females (5 days after emergence, as well as 7 and 12 days after they fed upon uninfected jirds) synthesized methionine at a rate of 0.6 nmole per hr per mg protein. Extracts of female mosquitoes prepared 7 and 12 days after they fed upon Brugia pahangi-infected jirds synthesized methionine at double the normal rate. Because methionine formation by extracts of adult B. pahangi could not be detected, it is probable that methionine synthetase activity increased in the arthropod host in response to filarial infection.