Activity in ~he presenc~ of L-v~ne methy] ease, a~fl of L-3-arn~no-4-me~hylp~n1~.~-2-one (rne~yLk,et.~ne) WaS ~ea~ured in the ~am~ way. inhibitionThe ra~,e of ~rever~ib]e kr--~,a'b~-~fon was f~owed by assaying ~e~tty ,(as in 2~.) :an s~anp]e~ at diff~ren~ time~ zfie:r mixing enzyme and ~ ~uh~:bitcr. The ~bitor~, ch]or,orneihyLketones, L-valhae mad L-aspaztme aua]ogUes were incubated w~t~ enzyme ira 0.05 ~M K-phosphate . buffer, pH 7.5 at 25~C. The kine dc constants of 5,~bAbi-don were de;retained v,5 ~th a ] O.0-foid concenlradon ex~.ess of ]nh~bitox o~er e~az3cme. purified beef liver valyl-tRNA syn~et,'~e was oblzSn 2Ao Test for pos~ble reae~q~arion by he p~:o~edure ha ~7] with naoditSca~tioJ~s t,o be " Enzyme (3 n~g/an])was 5a~:mabmed for 5 h~-Lu ~lhe descr~bed ,~lsewhe~.-The preparation eontained ~raeez prose-~c,e o~ 5 X-a:0 -4 aM ~h]0r0rnOllaytke~one or:..~ethyi ofsome other synthetasez anfl.!some olheI pxo~e.ins, .ketone a~ in 2,3, A]iquots were :ddluled 100-fold and

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