1. The stability of enzyme activity of the creatine kinase-l-immunoglobulin-G complex has been determined at 37°C, 4°C and −20°C in heat-inactivated serum and buffer.

2. The complex was formed by incubating creatine kinase-1 and immunoglobulin G at 37°C for 30 min. It was isolated by Sephadex G-100 column chromatography.

3. At all temperatures the complex suspended in buffer was about twice as stable as it was in heat-inactivated serum. Stability decreased in the sequence of 4°C, 37°C and −20°C. Therefore all isolations of the complex were carried out at 4°C.

4. At 37°C the decay of enzyme activity of the complex was found to be biphasic first order. In serum the Kd values were −0·045 min−1 and −0·0028 min−1, and in buffer −0·023 min−1 and −0·0016 min−1.

5. From column-chromatography experiments it was found that between 10 and 20% of the total creatine kinase-1 was involved in the complexing reaction after a 30-min incubation.

6. With this 10·20% proportion it can be calculated that the overall t0·5 for the decay of total creatine kinase-1 activity must be in the range 95–201 min. This finding suggests, by comparison with other published data, that the enzyme—immunoglobulin complex is the main route of creatine kinase-1 catabolism in serum.

7. The difference between serum and buffer decay values for the complex is possibly due to the presence of cystine, urate and other substances in serum, which are additional potent creatine kinase inhibitors.

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