Abstract
Abstract An aminoacyltransferase preparation has been obtained from a beef liver homogenate, and a 44-fold purification of the activity has been achieved. The enzyme preparation catalyzes both peptide synthesis from suitable amino acid esters (e.g. phenylalanine methyl ester, tyrosine methyl ester, leucine methyl ester) and the hydrolysis of such esters, the former process predominating at pH values near 7. Both actions are completely inhibited by diisopropyl phosphorofluoridate. Evidence is presented to show that in the formation of dipeptides (e.g. phenylalanylphenylalanine) from amino acid esters (e.g. phenylalanine methyl ester), the corresponding dipeptide ester is an intermediate. In peptide synthesis, the enzyme preparation exhibits side chain specificity with respect to both the acyl donor and the amine acceptor, but the specificity with respect to the amine acceptor appears to be more restricted, and limited to esters of certain L-amino acids. Amino acid amides and peptide amides corresponding to susceptible esters are resistant to enzymic action.
Highlights
We describe an enzyme preparation that is an effective catalyst of the synthesis of peptides from suitable amino acid esters and that catalyzes the hydrolysis of such esters
Action on Amino Acid Esters and Amides-It will be seen from Table I that the procedure developed for the concentration of the aminoacyltransferase activity of beef liver led to a 44-fold purification, with L-leucine methyl ester as a substrate at p 7
During the course of the preparation, each of the seven fractions listed in Table I was assayed for esterase activity toward methyl butyrate at pH 7, and it was found that the ratio of the initial rate of hydrolysis of this substrate to the reaction of L-leucine methyl ester remained relatively constant (0.60 i 0.09)
Summary
An aminoacyltransferase preparation has been obtained from a beef liver homogenate, and a 44-fold purification of the activity has been achieved. The enzyme preparation catalyzes both peptide synthesis from suitable amino acid esters (e.g. phenylalanine methyl ester, tyrosine methyl ester, leucine methyl ester) and the hydrolysis of such esters, the former process predominating at pH values near 7 Both actions are completely inhibited by diisopropyl phosphorofluoridate. We describe an enzyme preparation (from beef liver) that is an effective catalyst of the synthesis of peptides from suitable amino acid esters and that catalyzes the hydrolysis of such esters. We recognize that this term has been applied to some of the "transfer factors" that promote the transfer of '4 C-labeled aminoacyl units of aminoacyl-sRNA to trichloracetic acid-insoluble material [5], but are unable at present to suggest a more appropriate designation for the enzymic activity described in this communication
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