ISSN:
1432-136X
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Medicine
Notes:
Summary The apparent Michaelis constant (K m) of NADH for muscle-type (M4 isozyme) lactate dehydrogenases (LDHs) is highest, at any given temperature of measurement, for LDHs of cold-adapted vertebrates (Table 1). However, these interspecific differences in theK m of NADH are not due to variations in LDH-NADH binding affinity. Rather, theK m differences result entirely from interspecific variation in the substrate turnover constant (k cat) (Fig. 1; Table 2). This follows from the fact that theK m of NADH is equal tok cat divided by the “on constant” for NADH binding to LDH,k 1, so that interspecific differences ink cat, combined with identical values fork 1 among different LDH reactions, make the magnitude of theK m of NADH a function of substrate turnover number. The temperature dependence of theK m of NADH for a single LDH homologue is the net result of temperature dependence of bothk cat andk 1 (Figs. 3 and 4). Temperature independentK m values can result from simultaneous, and algebraically offsetting, increases ink cat andk 1 with rising temperature. Salt-induced changes in theK m of NADH also may be due to simultaneous perturbation of bothk cat andk 1 (Table 3). These findings are discussed from the standpoint of the evolution of LDH kinetic properties, particularly the interspecific conservation of catalytic and regulatory functions, in differently-adapted species.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00689209
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