Creatine kinase in serum: 5. Effect of thiols on isoenzyme activity during storage at various temperatures.

We studied changes in the activity of human creatine kinase isoenzymes (native MM in serum, MM, MB, and BB preparations in serum matrix) on storage in the presence of 0, 10, 20, and 50 mmol of N-acetyl cysteine, 2-mercaptoethanol, and 1-thioglycerol per liter of serum at 37, 30, 25, 4, and -20 degrees C. We also assessed the stability of the thiols under the same conditions. We generally confirmed increasing stability in the sequence: BB, MB, and MM isoenzyme. Above 30 degrees C irreversible inactivation is very rapid, even in the presence of thiols. At lower temperatures endogenous MM is sufficiently stable; thus, the use of protective thiols can be restricted to sera expected to contain isoenzymes MB or BB. Our studies lead us to prefer use of 50 mmol of N-acetyl cysteine per liter of serum. The other thiols cause turbidity to develop earlier and their decomposittion products diminish the activity more. The initial activity of the MB and BB preparations increased by as much as 2.3- and 1.8-fold on storage with thiols. This effect was not observed with either endogenous MB and MM or with the MM prepartion.