Creatine kinase and lactate dehydrogenase: stability of isoenzymes and their activity in stored human plasma and prostatic tissue extracts and effect of sample dilution.

We examined the effect of storing human plasma or extracts of prostate at -90 degrees C on the activity of creatine kinase and lactate dehydrogenase and isoenzyme distribution. Enzyme activities were unaltered during storage for as long as six weeks. If these preparations were thawed only once at 2 to 4 degrees C, they could be stored for as long as 165 days at -90 degrees C with no change in isoenzyme distribution. Inexplicably, apparent isoenzyme distribution of prostatic lactate dehydrogenase was sensitive to sample dilution, whereas the isoenzyme distribution of lactate dehydrogenase in plasma was not. Our observations emphasize the importance of validating details of analytical protocols that are to be used for quantification of new types of specimens.