Scientia Sinica

80 SCIENTIA SINICA Vol. V

Table 4, Stability of Succinic Dehydrogenase under Various pH.

A partially purifed enzyme preparation was’ kept under vacuum at 0°C for 64 hours. The pH value of the medium during incubation was as indicated below. At the end of the incubation period, the remaining activity was assayed and expressed as % of the original.

a

pH During Incubation Succinate Concentration, M Remaining Activity, % of Original 5.6 S= 3 5.6 0.04 93 6.5 . =o 63 Ae Or : = ; 100 8.7 = 70 8.7 0.04 100 ee) — 4 ORD) 0.04 100

rr 5,

The activated enzyme loses some activity when succinate is removed by dialysis under vacuum and can be re-activated by succinate.

Table 5. Activation. of Succinic Dehydrogenase by Succinate.

The enzyme was incubated with 0,04 M succinate at 0°C under nitrogen and at the times indicated below aliquots were withdrawn and assayed with ferricyanide. Activities are expressed as percentages of the activity before the addition of succinate.

Activity

Incubation Time Heart Muscle Preparation | Purified Enzyme (specific activity=64)

0 100 100 10 min. 120 30 min. 107 151 3.5 br. 182 24 hr. 273

Effect of phosphate on enzyme activity. The activity of the solubilized enzyme is not significantly changed when the phosphate concentration in the reaction mixture is changed from 0.05 M to 0.15 M. If borate or imidazole buffer is used instead of phosphate buffer, the activity of the dehydrogenase is only slightly lower. The phosphate concentration during reaction in borate or imidazole buffer was, by direct determination, only 3.8 X 10° M. If 0.05 M phosphate is also present, the enzyme activity is the same as in phosphate