Synthesis of ubiquinone 9 by adult Brugia pahangi and Dirofilaria immitis: evidence against its involvement in the oxidation of 5-methyltetrahydrofolate.

Among various ubiquinone (Q) isoprenologues tested, only Q7 was more efficient than menadione in promoting the oxidation of 5-methyltetrahydrofolate (CH3FH4) by 5,10-methylenetetrahydrofolate reductase isolated from adult Brugia pahangi, whereas Q10 was the best cofactor in the same reaction catalysed by the analogous enzyme from adult Dirofilaria immitis. Menoctone (3-[1-cyclohexyloctyl] -2-hydroxy-1,4-naphthoquinone) was a strong competitive inhibitor of both these ubiquinone isoprenologues in the respective reactions. When incubated in the presence of D,L-[14C]-mevalonate, adult B. pahangi and D. immitis synthesized radiolabelled Q9 only, in addition to other isoprenoid derivatives in the neutral lipid fraction. In view of the inability of Q9 to promote the oxidation of CH3FH4 by 5,10-methylenetetrahydrofolate reductase from B. pahangi, it seems unlikely that this filaria uses Q9 as a cofactor in this reaction. Conceivably, D. immitis could use Q9 as a cofactor in its enzymatic oxidation of CH3FH4, since in this circumstance, it was a better cofactor than menadione.