1. An activated form of PTC (PTC') which incorporates the properties of both native PTC and activated PTA (PTA') has been identified in BaSO4 eluates of intact citrated plasma.
2. PTC' has been synthesized from partially purified reagents in a reaction shown to require only PTA' and native PTC. Calcium ions greatly accelerate this reaction, but PTC' synthesis occurs slowly in their absence.
3. Kinetic and chromatographic analyses have shown that PTA' functions as an enzyme catalyzing the conversion of native PTC to PTC'.
4. On DEAE-cellulose chromatography, PTC and PTC' were found to elute at the same salt concentration.