Co2+
divalent metal ion is required, lower activity than Mg2+ or Mn2+
Mg2+
divalent metal ion is required, Zn2+ or Ni2+ are ineffective
Mg2+
Mn2+ (32% relative activity) and Ca2+ (61% relative activity) can substitute for Mg2+
Mn2+
divalent metal ion is required, higher activity than Mg2+. Zn2+ or Ni2+ are ineffective