Characterization of cardiac troponin subunit release into serum after acute myocardial infarction and comparison of assays for troponin T and I. American Association for Clinical Chemistry Subcommittee on cTnI Standardization

We examined the release of cardiac troponin T (cTnT) and I (cTnI) into the blood of patients after acute myocardial infarction (AMI). Three postAMI serum samples were applied in separate analytical runs onto a calibrated gel filtration column (Sephacryl S-200), and the proteins were separated by molecular weight. Using commercial cTnT and cTnI assays measured on collected fractions, we found that troponin was released into blood as a ternary complex of cTnT-I-C, a binary complex of cTnI-C, and free cTnT, with no free cTnI within the limits of the analytical methodologies. The serum samples were also examined after incubation with EDTA and heparin. EDTA broke up troponin complexes into individual subunits, whereas heparin had no effect on the assays tested. We added free cTnC subunits to 24 AMI serum samples and found no marked increase in the total cTnI concentrations, using an immunoassay that gave higher values for the cTnI-C complex than free cTnI. To characterize the cross-reactivity of cTnT and cTnI assays, purified troponin standards in nine different forms were prepared, added to serum and plasma pools, and tested in nine quantitative commercial and pre-market assays for cTnI and one approved assay for cTnT. All nine cTnI assays recognized each of the troponin I forms (complexed and free). In five of these assays, the relative responses for cTnI were nearly equimolar. For the remainder, the response was substantially greater for complexed cTnI than for free cTnI. Moreover, there was a substantial difference in the absolute concentration of results between cTnI assays. The commercial cTnT assay recognized binary and ternary complexes of troponin on a near equimolar basis. We conclude that all assays are useful for detection of cardiac injury. However, there are differences in absolute cTnI results due to a lack of mass standardization and heterogeneity in the cross-reactivities of antibodies to various troponin I forms.

Protonation and free energy changes associated with formation of QBH2 in native and Glu-L212-->Gln mutant reaction centers from Rhodobacter sphaeroides

Formation of the quinol QBH2 in Glu-L212-->Gln mutant [EQ(L212)] reaction centers (RCs) from Rhodobacter sphaeroides was investigated by measuring the proton uptake (using dyes), UV absorption changes, and free energy changes associated with the two-electron reduction of QB. The advantage of using the EQ(L212) RCs for these studies is that the individual protonation steps can be kinetically resolved and analyzed; conclusions reached regarding the mechanism of formation of QBH2 are expected to apply also to native RCs. The proton uptake by EQ(L212) RCs was strongly biphasic: the fast phase was essentially concomitant with the second electron transfer to QB- (approximately 1 ms at pH 7.5); the slow phase was approximately 2000-fold slower. The rate constant of the slow phase depended on the redox state of the primary quinone QA; for QA- the rate constant was larger (i.e., 8-fold at pH 6.0) than for QA. The electron and proton transfers to QB- in EQ(L212) RCs were modeled with a two-step scheme as follows: (1) fast, QA-QB- + H+(1)-->QA(QBH)-; (2) slow, QA(QBH)- + H+(2)-->QAQBH2, where reaction 1 involves concomitant electron transfer and proton uptake [Paddock, M. L., McPherson, P. H., Feher, G., & Okamura, M. Y. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 6803-6807]. The stoichiometry of the fast proton uptake associated with the two-electron reduction of QB varied from 1.1 to 1.4 H+/2e- at pH 6.5-8.5, consistent with the uptake of H+(1) plus an additional fractional proton uptake due to amino acid residues whose pKa values are shifted by interactions with the charge of (QBH)-. The total steady-state proton uptake stoichiometry was 2.0 H+/2e- at pH < or = 7.5, consistent with the formation of the quinol QBH2 (reactions 1 and 2). At pH 8.5, the steady-state proton uptake was 1.6 +/- 0.1 H+/2e-, which is consistent with an apparent pKa for H+(2) of approximately 8.5 [McPherson, P. H., Okamura, M. Y., & Feher, G. (1993) Biochim. Biophys. Acta 1144, 309-324]. The proton uptake kinetics indicate that Glu-L212 is a component of the proton transfer chain for H+(2) that connects reduced QB (buried in the RC protein) to the aqueous solvent as proposed previously [Paddock, M. L., Rongey, S. H., Feher, G., & Okamura, M. Y. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 6602-6606].(ABSTRACT TRUNCATED AT 400 WORDS)

Pathway of proton transfer in bacterial reaction centers: role of aspartate-L213 in proton transfers associated with reduction of quinoneto dihydroquinone

The role of Asp-L213 in proton transfer to reduced quinone QB in the reaction center (RC) from Rhodobacter sphaeroides was studied by site-directed replacement of Asp with residues having different proton donor properties. Reaction centers (RCs) with Asn, Leu, Thr, and Ser at L213 had greatly reduced (approximately 6000-fold) proton-coupled electron transfer [kAB(2)] and proton uptake rates associated with the second electron reduction of QB (QA- QB- + 2H(+)-->QAQBH2) compared to native RCs. RCs containing Glu at L213 showed faster (approximately 90-fold) electron and proton transfer rates than the other mutant RCs but were still reduced (approximately 70-fold) compared with native RCs. These results show that kAB(2) is larger when a carboxylic acid occupies the L213 site, consistent with the proposal that Asp-L213 is a component of a proton transfer chain. The reduced kAB(2) observed with Glu versus Asp at L213 suggests that Asp at L213 is important for proton transfer for some other reason in addition to its proton transfer capabilities. Glu-L213 is estimated to have a higher apparent pKa (pKa > or = 7) than Asp-L213 (pKa < or = 4), as indicated by the slower rate of charge recombination (D+QAQB(-)-->DQAQB) in the mutant RCs. The importance of the pKa and charge of the residue at L213 for proton transfer are discussed. Based on these studies, a model for proton transfer is proposed in which Asp-L213 contributes to proton transfer in native RCs in two ways: (1) it is a component of a proton transfer chain connecting the buried QB molecule with the solvent and/or (2) it provides a negative charge that stabilizes a proton on or near QB.

Light-induced proton uptake by photosynthetic reaction centers from Rhodobacter sphaeroides R-26.1. II. Protonation of the state DQAQB2-

Proton uptake associated with the two-electron reduction of QB was investigated in reaction centers (RCs) from Rhodobacter sphaeroides R-26.1 using pH-sensitive dyes. An uptake of two protons was observed at pH < or = 7.5, consistent with the formation of the dihydroquinone QBH2. At higher pH, the proton uptake decreased with an apparent pKa of approx. 8.5, i.e., to 1.5 H+/2 e- at pH 8.5. A molecular model is presented in which the apparent pKa is due to the protonation of either the carbonyl oxygen on QB or of an amino acid residue near QB (e.g., His-L190). Experimental evidence in favor of the protonation of the oxygen is discussed. The kinetics of the electron transfer from QA-QB- to QAQB2- and the associated proton uptake were compared at several pH values and temperatures. At pH 8.5 (21.5 degrees C) the rate constants for the proton uptake and electron transfer are the same within the precision of the measurement. At lower pH, the proton uptake rate constant is smaller than that for electron transfer. The difference between the rate constants is temperature dependent, i.e., it varies from 12 +/- 4% at 21.5 degrees C (pH 7.5) to 28 +/- 4% at 4.0 degrees C (pH 7.5). We show that the kinetics can be explained by a previously proposed model (Paddock, M. L., McPherson, P. H., Feher, G. and Okamura, M. Y. (1990) Proc. Natl. Acad. Sci. USA 87, 6803-6807) in which the uptake of two protons by doubly reduced QB occurs sequentially, one concomitant with and the other after electron transfer.

Pathway of proton transfer in bacterial reaction centers: replacement of serine-L223 by alanine inhibits electron and proton transfers associated with reduction of quinone to dihydroquinone

The pathway of proton transfer in the reaction center (RC) from Rhodobacter sphaeroides was investigated by site-directed mutagenesis. Ser-L223, a putative proton donor that forms a hydrogen bond with the secondary quinone acceptor QB, was replaced with Ala and Thr. RCs with Ala-L223 displayed reduced electron transfer and proton uptake rates in the reaction QA-QB- + 2H+----QAQBH2. The rate constant for this reaction, k(2)AB, was found to be reduced approximately 350-fold to 4.0 +/- 0.2 s-1. Proton uptake measurements using a pH indicator dye showed a rapid uptake of 1 H+ per RC followed by a slower uptake of 1 H+ per RC at a rate of 4.1 +/- 0.1 s-1; native RCs showed a rapid uptake of 2H+ per RC. Evidence is provided that these changes were not due to gross structural changes in the binding site of QB. RCs with Thr-L223 showed little reduction in the rates of electron and proton transfer. These results indicate that proton transfer from the hydroxyl group of Ser-L223 or Thr-L223 is required for fast electron and proton transfer associated with the formation of the dihydroquinone QH2. In contrast, previous work showed that replacing Glu-L212, another putative proton donor to QB, with Gln slowed proton uptake from solution without significantly altering electron transfer. We propose a model that involves two distinct proton transfer steps. The first step occurs prior to transfer of the second electron to QB and involves proton transfer from Ser-L223. The second step occurs after this electron transfer through a pathway involving Glu-L212.