The effects of N-ethylmaleimide on active amino acid transport in Escherichia coli.

N-Ethylmaleimide (MalNEt) binds covalently and without specificity to accessible sulfhydryl residues in proteins. In some cases specificity has been imposed on this reaction by manipulating reaction conditions, yielding information concerning both enzyme mechanism and the identity of specific proteins (for example C.F. Fox and E.P. Kennedy (1965) Proc. Natl. Acad. Sci. u.s. 54, 891-899) and R.E. McCarty and J. Fagan (1973) Biochemistry 12, 1503-1507). We have examined the effects of MalNEt on the active accumulation of nine amino acids by Escherichia coli strains ML 308-225 and DL 54. Whole cells have been used in order that transport systems both dependent on and independent of periplasmic binding proteins could be studied under various conditions of energy supply for transport. Our results suggest that the systems transporting ornithine, phenylalanine and proline are those most likely to undergo inactivation by direct reaction of MalNEt with the transport apparatus, rather than merely via side effects such as interruption of their energy supply. The inhibition of proline transport is specifically enhanced by the presence of proline, competitive inhibitors of proline transport, or carbonylcyanide p-trifluoromethyoxyphenylhydrazone during MalNEt treatment. The other eight systems tested showed no analogous effects.