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The easiest way to effect a change in rate, as measured by the accumulation of a reaction product, is to alter the concentration of reactants. In the case of an enzymatic reaction, if we assume a constant concentration of substrate and increase the amount of enzyme, the product of the reaction will accumulate more rapidly: an increase in activity. Surprisingly, few studies quantitatively address this issue. Most studies simply assume that short‐term changes in enzyme activities are due to enzyme concentration changes. The one study, consistently cited, that does address changes in enzyme activity as a function of enzyme concentration is Sidell et al. (1973). Figure 2.8 shows the activity of cytochrome oxidase, an important enzyme in the electron transport system, from goldfish skeletal muscle in fishes first acclimated to 15 °C and then transferred to either 5 or 25 °C. The enzyme activity per milligram protein in both groups was then monitored for approximately 30 days. Activity was much higher in the fishes transferred to the colder temperature, suggesting that the concentration of enzyme was much higher in the cold‐adapted fish.

It should be noted that accelerating or decelerating the activity of an enzymic pathway through differences in enzyme concentrations is best as a short‐term solution or for small T°C changes. It is not practical for a long‐term (evolutionary) solution to synthesize larger quantities of an inefficient enzyme to deal with the cold.