The three pyridylalaiiirie S-oxides, 8-(2-, 3-, or 4-pyridgl l-osidej-~~i,-al:triirie, have been synthesized, :rii([ their biological activities in Wcherrchia coli 9723 and Lactobacillus arahinosus 17-5 have been determined. 4-Py1,idglalanine N-oxide inhibits growth of E. coli at roiicentratiotii of :30 p g , irrl aiid higher. The 2-anti 3-pyridylalanine S-oxides are less efl'ective inhibitors in this orgallism: the tosicsit ies of all three N-oxides are reversed 1)y supplements of phenylalanine or tyro.iine. Evidence is giveii for the enzymatic redirct ion of the p;ridgIala,riitir S-oxides to the corresponding pyridylalariines in E. coli and for the probability that the toxicity of 4-pyridylalanine N-oxide in this organism is due t>o the formation of t,he more toxic 4-pyridylalailine. The pyridylrtlniiiiie N-oxides are inhibitory to I,. arabinosus only at very high c,orirentrations : the organism does not reduce t h i w compounds to the pyridylalanines.
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