Graduate Thesis Or Dissertation
 

Thermal inactivation studies on some enzymes from Vibrio marinus, an obligately psychrophilic marine bacterium

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  • Some enzymes of the citric acid cycle and glycolytic pathway in cell-free extracts of Vibrio marinus MP-1 were compared for thermal lability. After one hour of moderate temperature exposure, enzymes of both pathways rapidly lost catalytic activity. For all but one enzyme, 50 percent remaining activity occurred near an averaged temperature of 27 C. Succinic dehydrogenase was remarkably unstable, being 50 percent inactivated at 16 C. Complete loss of enzymatic activity for the TCA and glycolytic enzymes took place at an averaged temperature of 32 C. Lactic dehydrogenase was the most thermostable as evidenced by activity up to 39 C. Several of the "psychrophilic" enzymes (hexokinase, aldolase, and lactic dehydrogenase) produced greater activity in the presence of sodium chloride as opposed to tris-HC1. Suspending the enzyme preparations in sodium chloride resulted in an increased thermal stability. The 50 percent remaining activity took place at 1 C to 15 C higher with an average of 32 C. Complete loss of catalytic function averaged 2 C higher for the sodium chloride suspended enzyme. There was noticeably less difference in thermal protection by sodium chloride at temperatures above the region of 50 percent remaining activity. The effects of salt are explained on the basis of increased activity coefficients in solution. The behavior of lactic dehydrogenase on moderate temperature exposure suggested the existence of isozymes, or that denaturation occurred in three major steps, each with a different rate. The low temperature requirement of all the enzymes under investigation, together with the ionic requirement of hexokinase and the relatively high level of phosphoglucose isomerase were suggested as possible factors controlling the obligate psychrophile's activities. Furthermore, the diverse influence of moderate temperature exposure on each enzyme (whether in buffer of sodium chloride) could alter differentially the cell's catalytic processes so that they could no longer operate in synchrony. Comparison with enzymes from other sources indicated the tricarboxylic acid cycle and glycolytic enzymes of Vibrio marinus MP-1 to be conspicuously thermal labile.
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