Temperature dependence of kinetic parameters of (Ca2+ + Mg2+)-ATPase in rabbit and winter flounder sarcoplasmic reticulum.

The effect of temperature on the activity of (Ca2+ + Mg2+)-ATPase in rabbit (R) and winter flounder (F) sarcoplasmic reticulum (SR) has been investigated. The enzymes from the two sources appear to be differently adapted to temperature. (Ca2+ + Mg2+)-ATPase activity of FSR was high at 35 degrees C, and declined to zero at 40 degrees C, whereas RSR was still active above 45 degrees C. Km for ATP increased with temperature in a biphasic fashion for both enzymes. The Km values for FSR were 69-75 microM at lower temperatures (9-18 degrees C) and 80-187 microM at higher temperatures (18-35 degrees C). Values for RSR were 6.5-13 microM below 32 degrees C and 37-186 microM above this temperature. At their respective physiological temperatures the enzyme from both rabbit and flounder exhibited similar Km (70-80 microM). Effective ATP binding enthalpies were 3-5 times lower for FSR than for RSR in both temperature regions. Binding energies increased with temperature 3-4 fold for enzyme in both SR. The enzyme in FSR is suggested to be a more effective catalyst than the one in RSR in the sense that its activation energy for ATP hydrolysis is lower. These variations may arise from dissimilarities in either the protein, or in its surrounding lipid, or both. RSR and FSR are significantly different in the nature of the unsaturated chains in their constituent lipids. The difference in lipid composition might account for some of the deversity in the kinetic parameters.