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SOME PROPERTIES OF A HOMOCARNOSINE‐CARNOSINE SYNTHETASE ISOLATED FROM RAT BRAIN 1
Author(s) -
Skaper S. D.,
Das S.,
Marshall F. D.
Publication year - 1973
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1111/j.1471-4159.1973.tb06027.x
Subject(s) - carnosine , dipeptide , enzyme , histidine , biochemistry , chemistry , substrate (aquarium) , deamidation , hydrolysis , amino acid , stereochemistry , biology , ecology
—An enzyme from rat brain catalysing the synthesis of the histidine‐containing dipeptides carnosine and homocarnosine ( l .‐histidine: β‐alanine ligase (AMP) [EC 6.3.2.11]) was purified about 30‐40‐fold from a 100,000 g supernatant. Assays were conducted by measuring the incorporation of L‐[14C]histidine into carnosine and homocarnosine isolated by paper electrophoresis from the incubation mixture. The ratios of specific activities for the formation of carnosine and homocarnosine were not significantly different for the various purification steps. This was taken as evidence of one enzyme synthesizing both dipeptides. In studying the properties of this enzyme, a pH optimum of 7.4 was shown for carnosine synthesis. The concentrations of amino acid substrates giving maximal synthesis of both dipeptides were in the physiological range found for rat brain. An apparent requirement for ATP, Mg 2+ , and DPN was seen for dipeptide synthesis. A substrate dependent, enzymecatalysed 32 PPi‐ATP exchange reaction was observed, suggesting the formation of an aminoacyl‐AMP intermediate. Certain other nucleoside triphosphates could substitute for the ATP; this effect showed a specificity toward the dipeptide being synthesized. The apparent requirement for DPN was quite specific, with a number of related compounds having no effect. The stoichiometry of enzyme‐catalysed carnosine synthesis was studied. A one to one relationship between carnosine formed and ATP hydrolysed was demonstrated. However, the ratio between carnosine synthesized and DPN hydrolysed was about 6 to 1, indicating a catalytic role for the DPN. The breakdown of DPN did not occur with enzyme alone but was dependent on the presence of substrate.