Creatine kinase in serum: 4. Differences in substrate affinity among the isoenzymes

The goal of this work was to find out whether it is possible to measure all three creatine kinase isoenzymes under the same reaction conditions in spite of their different kinetic properties. We found the tightest substrate binding for purified human BB, followed by the MB And MM isoenzyme preparations for both creatine phosphate and ADP. An increase in substrate concentration usually resulted in an inhibition. Nevertheless, it was possible with a method optimized for the MM isoenzyme also to measure the BB and MB isoenzymes at a rate of inhibition of only 6 and 3%, respectively. Marked differences in the apparent Km values between purified and native MM isoenzyme in human serum may indicate that the enzyme declined in substrate affinity during the isolation procedure. The use of enzyme preparations for standardization purposes, therefore, is only suitable if their kinetic properties are close to those of the enzyme in serum. Difficulties in the calculation of the apparent Km values are discussed and the graphical procedures of Lineweaver and Burk and of Eisenthal and Cornish-Bowden compared.