DIFFERENTIAL EXPRESSION AND ACTIVITY OF FLAVIN-CONTAINING MONOOXYGENASES IN EURYHALINE AND STENOHALINE FLATFISHES INDICATES POTENTIAL OSMOREGULATORY ROLE

N,N-Dimethylaniline (DMA) N-oxidase activity indicative of flavin-cont aining monooxygenase (FMO) was examined in four tissues (liver, gill, muscle, and kidney) of the flounder (Platichthys flesus). Gill microso mes had the highest levels of activity (456 +/- 343 pmol/min/mg), whil e kidney (121 +/- 109) and liver (67 +/- 26) had levels just above det ection, A single faint band of a 56 kD protein was observed in liver a nd gill microsomes following Western blot analyses with polyclonal ant ibodies to FMO 1, DMA N-oxidase activity in gill and liver directly co rrelated with the expression of the 56 kD protein recognized by polycl onal antibodies against FMO form 1, Like,vise a mRNA band of approxima tely 2.5 kilobases was higher in gill than a 3.0 kb band in liver foll owing hydridization with an FMO 1 cDNA probe, Gill and liver microsoma l DMA N-oxidase from the euryhaline P. flesus was compared with that o f the stenohaline turbot (Scophthalmus maximus), DMA oxidase activity, FMO protein and mRNA were significantly greater in the gill of P. fle sus, while S, maximus had higher levels of enzyme activity in the live r, but also significant levels in gill, Comparison of the enzymatic pr operties of the P. flesus gill and S. maximus liver enzymes indicated dramatic differences in K-m between gill and liver, but were both inhi bited by equimolar concentrations of trimethylamine (TMA). Gill micros omal activity in each species was unaffected by the mammalian FMO 2 su bstrate (competitive inhibitor) n-octylamine. Differential expression of FMO in tissues from stenohaline and euryhaline fish suggests a func tional relationship between FMO and osmoregulation.