Two pathways are possible during the proteolytic formation of alpha-thrombin (alpha-IIa) from prothrombin (II) or prethrombin 1 (P1). One of the pathways, with prethrombin 2 or prethrombin 2 associated with fragment 2 (P2F2) as intermediates, has long been known to exist when activation is catalyzed by Factor Xa (Xa) alone. The second pathway, with meizothrombin or meizothrombin (des fragment 1) (MzIIa(-F1)) as intermediate, has been shown to exist when Factor Va and phospholipids are present with Xa. Until now, MzIIa(-F1) has not been detected in reactions catalyzed by Xa alone. In this study, we demonstrate that P1 activation by Xa alone occurs via both pathways, and we provide rate constants and kinetic equations for calculating the relative contributions of each of the pathways to the formation of alpha-IIa by Xa. Investigation of the initial rates of proteolytic cleavage of P2F2 and P1 by Xa alone indicated first-order dependence on substrate concentration with no evidence of saturation of Xa with either substrate at concentrations as high as 200 microM. Apparent second-order rate constants (kc/Km) of 113 +/- 9 M-1 s-1 for the formation of thrombin from P2F2 and 1,410 +/- 19 M-1 s-1 for the disappearance of P1 were determined at pH 7.5, 25 degrees C, 10 mM CaCl2, 0.15 M ionic strength. A two-step sequential first-order pathway employing these rate constants for thrombin activity production from P1 via P2F2 could not, however, account for the quantity of thrombin that was produced during the early stages of P1 activation. Addition of a parallel first-order reaction to produce thrombin activity from P1 independently of P2F2, tentatively identified as the formation of MzIIa(-F1), yielded progress curves in quantitative agreement with the experimental data. kc/Km for the parallel reaction was estimated to be 98 +/- 10 M-1 s-1. Independent determination of the second-order rate constant for the cleavage of isolated MzIIa (-F1), 15,000 +/- 420 M-1 s-1, indicated that MzIIa(-F1) could meet the kinetic requirements for an intermediate in the parallel activation pathway. The transient formation of MzIIa (-F1), as well as the generation of alpha-IIa, was directly demonstrated during activation of P1 by active site-affinity labeling of the reaction products with a biotin derivative of D-Phe-Pro-Arg chloromethyl ketone and visualization by semiquantitative Western blotting.(ABSTRACT TRUNCATED AT 400 WORDS)