Real-world evidence analysis of palbociclib prescribing patterns for patients with advanced/metastatic breast cancer treated in community oncology practice in the USA one year post approval

Background

Rapidly evolving understanding of cancer biology has presented novel opportunities to translate that understanding into clinically relevant therapy. Palbociclib, a novel, first-in-class cyclin-dependent kinase (CDK) 4/6 inhibitor was approved in the USA in February 2015 for the treatment of advanced/metastatic breast cancer. We examined real-world evidence in the first year post approval to understand the clinical and demographic characteristics of patients treated with palbociclib in community oncology practices and the dosing, treatment, and complete blood count (CBC) monitoring patterns.

Methods

This was a retrospective observational study of structured data from a US electronic medical record (EMR) database. Female patients receiving palbociclib after 31 January 2015 were followed through 31 March 2016. Our methodological rules were constructed to aggregate drugs received according to the order in which they are given, i.e., identify the line of therapy as first, second, or third line, etc., using treatment order and course description fields from the EMR.

Results

There were 763 patients initiating palbociclib who met the selection criteria. Of those, 612 (80.2%) received palbociclib concomitantly with letrozole. Mean follow up was 6.4 months and mean age at palbociclib initiation was 64 years. Of patients with a known starting dose (n = 417), 79.9% started on palbociclib 125 mg. Dose reductions were observed in 20.1% of patients. Percentages of patients according to line of therapy at initiation of palbociclib were first-line, 39.5%; second-line, 15.7%; third-line, 13.1%; and fourth-line therapy or later, 31.7%. On average, two CBC tests were conducted during the first cycle of palbociclib treatment. Overall, 74.6% of patients had a neutropenic event during follow up including 47.3% and 8.0% of patients with a grade 3 or 4 occurrence, respectively.

Conclusions

Real-world palbociclib use one year post US approval demonstrates a more heterogeneous patient population than that studied in the clinical trials with more than half of the patients receiving palbociclib plus letrozole in later lines of therapy. CBC testing rates suggested good provider compliance with monitoring guidelines in the USA prescribing information. The occurrence of grade 3 and 4 neutropenia (based on laboratory results) was consistent with the rates of grade 3 and 4 neutropenia in two phase-III studies (PALOMA-2, 56% and 10%; PALOMA-3, 55% and 11%, respectively). Understanding palbociclib utilization in real-world patients and how drug dosing and monitoring are performed aids in the understanding of safe and effective use of the drug.

Electronic supplementary material

The online version of this article (10.1186/s13058-018-0958-2) contains supplementary material, which is available to authorized users.

Abstract P6-16-05: Early utilization pattern of palbociclib 1 year post-approval in the United States

Background

Palbociclib was approved in the U.S. in February 2015 for the treatment of advanced/metastatic breast cancer (MBC) in combination with letrozole as initial endocrine based therapy for post-menopausal women with ER+/HER2- disease. We examined the demographic, clinical characteristics and treatment patterns of patients initiating palbociclib (PAL) + letrozole (LET) in real-world, community oncology practices.

Methods

This was a retrospective observational study of female breast cancer patients identified in the Navigating Cancer (NC) EMR database. The NC database collects EMR data, in both structured and unstructured fields (patient/clinical progress notes), from over 975 oncology and hematology providers across more than 50 locations in 25 states. Female patients with record of treatment with PAL after 01/31/2015 were selected. Combination treatment with LET was defined as having a record for LET within 30 days of the PAL prescription. Line of therapy (LOT) was assessed from the date of metastatic diagnosis and assigned by evaluating treatment plans pre-and post PAL initiation. Bi-monthly cohorts were constructed based on the month of initiation of PAL from 02/01/2015-01/31/2016. Interim results are presented; data from an additional three months of follow-up (through 03/31/2016) are pending.

Results

Overall, 931 unique patients were identified as having initiated PAL treatment. Of those, 608 (65.3%) received PAL + LET. Mean follow-up was relatively short at 5.4 mo (SD=3.5). Confirmed ER+/HER2- was observed in 71.6% of patients and 50.3% were age ≥65, mean age was 64.3 yrs. Of patients with available ECOG-PS at treatment initiation (n=424): 0/1=78.5%, 2=17.5% and 3=4.0%. Of patients with a known starting dose (n=418), 69.9% initiated with PAL 125mg, 22.0% at 100mg and 8.1% at 75mg. Compared to women < 65, women ≥ 65 were more often started with 100mg (25.4% vs. 18.9%) and 75mg (10.0% vs 6.5%). Any dose reductions were observed in 20.6% of patients (21.5% of patients receiving 125mg). During the year following approval, 39.8% of patients initiated PAL + LET at LOT1, 15.6% at LOT2, 13.0% at LOT3 and 31.6% at LOT4+, following MBC diagnosis. Over time the proportion of late use (LOT4+) declined from 39.7% in Feb/Mar '15 to 23.9% in Dec '15/Jan '16 with more patients utilizing in LOT3 (from 7.9% in Feb/Mar '15 to 19.5% in Dec '15/Jan '16).

Number of Patients Initiating PAL + LET by LOT and Month. AllFeb/Mar '15Apr/May '15Jun/Jul '15Aug/Sep '15Oct/Nov '15Dec'15/Jan '16N (%)608(100)63(10.4)108(17.8)125(20.6)108(17.8)91(14.5)113(18.6)LOT1 (%)242(39.8)23(36.5)45(41.7)53(42.4)45(41.7)33(36.3)43(38.1)LOT2 (%)95(15.6)10(15.9)12(11.1)19(15.2)15(13.9)18(19.8)21(18.6)LOT3 (%)79(13.0)5(7.9)10(9.3)18(14.4)12(11.1)12(13.2)22(19.5)LOT4+ (%)192(31.6)25(39.7)41(38.0)35(28.0)36(33.3)28(30.8)27(23.9)

Conclusions

There was a trend toward earlier utilization of PAL + LET from Feb-Jul '15, an increase in later use during Aug/Sep '15, and a return towards earlier use in subsequent cohorts reaching the lowest proportion of LOT 4+ use observed in Dec '15/Jan '16. After a mean follow-up of 5.4 mo, 21.5% of patients receiving the 125 mg dose had a dose reduction. Final results, with additional follow-up, will be presented at conference.

Citation Format: Kish JK, Ward MA, Garofalo D, Ahmed HV, McRoy L, Laney J, Zanotti G, Yu H-T, Feinberg BA. Early utilization pattern of palbociclib 1 year post-approval in the United States [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P6-16-05.

Patient (Pt)-reported symptoms of chemotherapy (chemo) and VEGFR/EGFR antibody therapies for the treatment of metastatic colorectal cancer (MCRC) in a U.S. community-based oncology practice network

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Background: Targeted therapies for MCRC have increased treatment (Tx) options in multiple therapy lines. Symptoms associated with MCRC Tx have not been well studied from the Pt perspective. Objective: To characterize symptoms reported by MCRC Pts treated with chemotherapy and/or targeted drugs at a large US oncology network.

Methods: Pt reported symptom data were linked to electronic medical records and Pt charts from Georgia Cancer Specialists in Southeast US. MCRC Pts aged ≥18 years with ≥1 administration of chemo or targeted therapy between 1/2007-3/2009 were included. Pt reports captured 13 symptoms, onset, and severity. Due to small sample sizes, no statistical comparisons were conducted.

Results: 332 MCRC Pts were included (median age 62 years, 47% male, median weight 74 kg, 48% ECOG PS 0 or 1). Amongst Pts receiving 1L Tx (n=299), 78% received bevacizumab (BV) +/- chemo, 4% cetuximab (CX) +/- chemo, 15% chemo only, and 2% other. 162 Pts received 2L Tx: 49% BV +/- chemo, 17% chemo only, 28% CX +/- chemo, and 6% other. The 5 most common symptoms in 1L Tx for all severity ratings were 36% fatigue, 20% nausea, 17% weight (wt) loss, 15% diarrhea, and 9% constipation. The 5 most common symptoms in all 2L Tx groups were 44% fatigue, 27% nausea, 19% diarrhea, 16% wt loss, and 11% abdomen pain. 2L moderate, severe, or disabling symptoms reported in >10% Pts receiving BV +/- chemo, CX +/- chemo, and chemo only Tx are shown in the table.

Conclusions: The most commonly reported symptoms by 1L and 2L MCRC Pts were fatigue, nausea, diarrhea, and weight loss. Overall, a numerically higher % of 2L Pts treated with CX +/- chemo reported moderate, severe, and disabling symptoms than Pts receiving BV +/- chemo and chemo alone. Rash was not observed in the BV +/- chemo group. Further research of MCRC Pt reported symptoms in a larger sample size are warranted.

[Table: see text]

[Table: see text]

Changing the treatment paradigm for patients with MDS: How the development of therapeutics influences population-based care in community oncology, the Georgia Cancer Specialists Outcomes Database experience

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Background: Clinical trials that lead to approval of new cancer therapeutics are based upon rigorous studies of safety & efficacy in a highly-restricted pt population. Community oncology practices are evolving to contribute to our understanding of the population- based effectiveness of these therapeutics by capturing and analyzing outcomes data. Georgia Cancer Specialists, a group practice of 41 hematologists/oncologists has been routinely tracking outcomes data since 1999. We now report outcomes for pts with MDS from our database. MDS was selected since this is a Medicare population, has typically been treated with supportive care only, and 3 new agents, 5-azacytadine (VID), lenalidomide (REV) and decitabine (DAC) have recently been approved allowing assessment of their impact on this cancer population. Methods: The Georgia Cancer Specialists Outcomes Database is an integrated electronic medical records and billing database that captures over 45 parameters on every pt visit at 31 sites. This study is a retrospective, observational study using de-identified pt-level information. Results: We followed 824 MDS pts from Jan-Oct, 2006, with 63% women; mean age 71 yrs; 61% Medicare/Medicaid. Only 7% of pts received at least one MDS-indicated chemotherapeutic. Of these, 44% received VID; 36% REV, and 43% DAC and 18% received at least two. At initial therapy, more REV pts (73%) were transfusion-dependent than VID (20%) and DAC (55%) (p=0.034). Average duration of observed therapy ranged from 2.3–3.25 cycles. A greater proportion of DAC pts (93%) had an improvement in RBC count within the first 4 cycles when compared to VID (44%) and REV (62.5%) (p<0.01). The treatment response profile for platelets was similar for VID and DAC. Conclusions: Practice patterns of elderly pts with MDS are changing as new therapies are adopted in community oncology practice. DAC appears to be very active, with the shortest time to initial response. Rigorous population based outcomes analyses at the practice level is an important component of quality cancer care and must supplement information generated by randomized clinical trials.

No significant financial relationships to disclose.

Using entropies of reaction to predict changes in protein stability: tyrosine-67-phenylalanine variants of rat cytochrome c and yeast Iso-1 cytochromes c

Using the voltammetric method of square-wave voltammetry, a direct electrochemical examination was made of the wild type and Tyr67Phe mutant of both rat cytochrome c and yeast iso-1-cytochrome c. In addition to determining the equilibrium reduction potential (E0') for each cytochrome, the entropy of reaction, deltaS0'(Rxn)(deltaS0'(Rxn) = S0'(Red) - S0'(Ox)), for the reduction process was determined via the non-isothermal method. Having determined deltaS0'(Rxn) and E0', deltaH0' was calculated. For rat cytochrome c, it was found that deltaS0'(Rxn) = -43 J mol(-1) K(-1) for the wild type and -53 J mol(-1) K(-1) for the Tyr67Phe variant, with the deltaH0' for both the wild type and variant nearly identical, indicating that the changes in reduction potential and probably stability are due to changes in deltaS0'(Rxn). In contrast the measured deltaS0'(Rxn) for yeast iso-1-cytochrome c demonstrated significant changes in both entropic and enthalpic contributions in going from wild type to mutant cytochrome c. The entropy of reaction provides information regarding the relative degree of solvation, and very likely the degree of compactness, of the oxidized state versus the reduced state of the redox protein. A thermodynamic scheme and stability derivation are presented that show how the entropies of reaction of wild type versus variant cytochromes contribute to and predict changes in stability in going from oxidized to reduced protein. For yeast iso-1-cytochrome c, the thermodynamically predicted change in stability was very close to the experimentally observed value, based on previous differential scanning calorimetric stability measurements. While such data is not available for rat cytochrome c, consideration of the enormously increased local stability of the rat oxidized cytochrome c variant predicts that the reduced rat variant will be even more stable than the already stabilized oxidized variant.

Direct voltammetric observation of redox driven changes in axial coordination and intramolecular rearrangement of the phenylalanine-82-histidine variant of yeast iso-1-cytochrome c

Direct square-wave and cyclic voltammetric electrochemical examination of the yeast iso-1-cytochrome c Phe82His/Cys102Ser variant revealed the intricacies of redox driven changes in axial coordination, concomitant with intramolecular rearrangement. Electrochemical methods are ideally suited for such a redox study, since they provide a direct and quantitative visualization of specific dynamic events. For the iso-1-cytochrome c Phe82His/Cys102Ser variant, square-wave voltammetry showed that the primary species in the reduced state is the Met80-Fe2+-His18 coordination form, while in the oxidized state the His82-Fe3+-His18 form predominates. The addition or removal of an electron to the appropriate form of this variant serves as a switch to a new molecular form of the cytochrome. Using the 2 x 2 electrochemical mechanism, simulations were done for the cyclic voltammetry experiments at different scan rates. These, in turn, provided relative rate constants for the intramolecular rearrangement/ligand exchange and the equilibrium redox potentials of the participating coordination forms: kb,AC = 17 s-1 for Met80-Fe3+-His18 --> His82-Fe3+-His18 and kf,BD > 10 s-1 for His82-Fe2+-His18 --> Met80-Fe2+-His18; E0' = 247 mV for Met80-Fe3+/2+-His18 couple, E0' = 47 mV for His82-Fe3+/2+-His18 couple, and E0' = 176 mV for the cross-reaction couple, His82-Fe3+-His18 + e- --> Met80-Fe2+-His18. Thermodynamic parameters, including the entropy of reaction, DeltaS0'Rxn, were determined for the net reduction/rearrangement reaction, His82-Fe3+-His18 + e- --> Met80-Fe2+-His18, and compared to those for wild-type cytochrome, Met80-Fe3+-His18 + e- --> Met80-Fe2+-His18. For the Phe82His variant mixed redox couple, DeltaS0'Rxn = -80 J/mol.K compared to DeltaS0'Rxn = -52 J/mol.K for the wild-type cyt c couple without rearrangement. Comparison of these entropies indicates that the oxidized His82-Fe3+-His18 form is highly disordered. It is proposed that this high level of disorder facilitates rapid rearrangement to Met80-Fe2+-His18 upon reduction.

Synthetic mutants of Clostridium pasteurianum ferredoxin: open iron sites and testing carboxylate coordination

The entire polypeptide chains for two new Clostridium pasteurianum ferredoxin (Fd) mutants were prepared with the following site-specific substitutions: Cys11Asp and Cys11 alpha-aminobutyric acid (Cys11 alpha-Aba), the latter being a non-naturally occurring amino acid. Standard t-Boc procedures were used for the synthesis and the peptides. The two apoproteins were reconstituted to the 2[4Fe-4S] holoprotein and their spectroscopic, redox and thermal properties were compared with those of native C.pasteurianum Fds. The fully reconstituted Cys11Asp and Cys11 alpha-Aba mutants were initially found to have both clusters intact, i.e. they were 2[4Fe-4S] ferredoxins. The unconventional ligands of Asp and alpha-Aba led to holo-Fds that were not very stable and easily released an iron to form the [3Fe-4S] cluster, presumably through oxidation. The Cys11 alpha-Aba mutant was somewhat more thermally stable than Cys11Asp. In contrast, while both mutants were less stable than the native protein upon exposure to oxygen, the Cys11 alpha-Aba mutant was less stable than Cys11Asp. The Cys11Gly mutant was also prepared, but all attempts, despite repeated and varied experimental conditions, at reconstitution to the Cys11Gly holo 2[4Fe-4S] Fd were unsuccessful, probably because a Gly-Gly sequence is known to break structure. This work, when compared with molecular biological site-specific mutagenesis, shows some of the advantages of chemical/in vitro reconstitution: certain mutants which cannot be detected as holoproteins by site-specific mutagenesis can be formed after all in vitro. Nonetheless, it seems apparent that altering any of the Cys coordination sites of the Fd clusters results in fundamentally more unstable ferredoxins.

Purification and properties of a low-redox-potential tetraheme cytochrome c3 from Shewanella putrefaciens

Shewanella putrefaciens is a facultatively anaerobic bacterium in the gamma group of the proteobacteria, capable of utilizing a wide variety of anaerobic electron acceptors. An examination of its cytochrome content revealed the presence of a tetraheme, low-redox-potential (E'o = -233 mV), cytochrome c-type cytochrome with a molecular mass of 12,120 Da and a pI of 5.8. The electron spin resonance data indicate a bis-histidine coordination of heme groups. Reduction of ferric citrate was accompanied by oxidation of the cytochrome. The biochemical properties suggested that this protein was in the cytochrome c3 group, which is supported by N-terminal sequence data up to the first heme binding site.

Solution structure of the oxidized 2[4Fe-4S] ferredoxin from Clostridium pasteurianum

Following the recently developed approach to the solution structure of paramagnetic high-potential iron-sulfur proteins, the three-dimensional structure in solution of the oxidized Clostridium pasteurianum ferredoxin has been solved by 1H-NMR. The X-ray structure is not available. The protein contains 55 amino acids and two [4Fe-4S] clusters. In the oxidized state, the clusters have S = 0 ground states, but are paramagnetic because of thermal population of excited states. Due to the somewhat small size of the protein and to the presence of two clusters, approximately 55% of the residues have at least one proton with a non-selective T1 smaller than 25 ms. The protein has thus been used as a test system to challenge the present paramagnetic NMR methodology both in achieving an extended assignment and in obtaining a suitable number of constraints. 79% of protein protons have been assigned. Analogy with other ferredoxins of known structure has been of help to speed up the final stages of the assignment, although we have shown that this independent information is not necessary. In addition to dipolar connectivities, partially detected through tailored experiments, 3JHN-H alpha, H-bond constraints and dihedral angle constraints on the Cys chi 2 angles have been generated by using a recently derived Karplus-type relationship for the hyperfine shifts of cysteine beta CH2 protons. In total, 456 constraints have been used in distance geometry calculations. The final quality of the structures is satisfactory, with root-mean-square deviation values of 66 pm and 108 pm for backbone and heavy atoms, respectively. The resulting structure is compared with that of Clostridium acidi urici ferredoxin [Duée, E. D., Fanchon, E., Vicat, J., Sieker, L. C., Meyer, J. & Moulis, J.-M. (1994) J. Mol. Biol. 243, 683-695]. The two proteins are very similar in the overall folding, secondary structure elements and side-chain orientations. The C alpha root-mean-square deviation values between the X-ray-determined C. acidi urici ferredoxin structure and the conformer with lowest energy of the C. pasteurianum ferredoxin family is 78 pm (residues 3-53). Discrepancies in residues 26-28 may arise from the disorder observed in the X-ray structure in that region.

Direct square-wave voltammetry of superoxidized [4Fe-4S]3+ aconitase and associated 3Fe/4Fe cluster interconversions

We report a direct square-wave voltammetric study of the iron-sulfur enzyme, aconitase, at the pyrolytic graphite edge electrode. New and established redox driven reactions were observed and the equilibrium reduction potential for each couple was determined: E0'[3Fe-4S]1+/0 = -268 mV, E0'[4Fe-4S]2+/1+ = -450 mV, E0'[4Fe-4S]3+/2+ = +100 mV, E0'Linear Form = -281 mV, and putatively, E0'[3Fe-4S]0/2- congruent to -1000 mV, all versus normal hydrogen electrode. Most importantly we have directly observed the superoxidized [4Fe-4S]3+ form of aconitase (originally proposed by Emptage, M. H., Dreyer, J.-L., Kennedy, M. C., and Beinert, H. (1983) J. Biol. Chem. 258, 11106-11111) and directly followed its conversion to the [3Fe-4S]1+ form; this intermediate is required for the deactivation of aconitase. Without exogenous ferrous iron, [3Fe-4S]0 aconitase is apparently super-reduced at very negative potentials to the [3Fe-4S]2- form and the concomitant formation of [4Fe-4S]2+ aconitase was followed over time. It is the apparent decomposition of super-reduced [3Fe-4S]2- aconitase that provides the source of ferrous iron for the interconversion of [3Fe-4S]0 aconitase to the [4Fe-4S]2+ form. Voltammetry of free and substrate bound [4Fe-4S]2+ aconitase showed that the latter is less susceptible to oxidation but, surprisingly, has the same E0'[4Fe-4S]3+/2+.

Direct electrochemical studies of hydrogenase and CO dehydrogenase

The reduction potentials of two relatively high-molecular-mass enzymes were determined directly at an edge pyrolytic graphite electrode by using square-wave voltammetry. The equilibrium reduction potential versus standard hydrogen electrode was determined for Clostridium pasteurianum hydrogenase I (E'0 = -377 +/- 10 mV; molecular mass 60 kDa) and Rhodospirillum rubrum carbon monoxide dehydrogenase (E'0 = -418 +/- 7 mV; molecular mass 62 kDa). The reduction potential of each enzyme was pH-independent, and one electron was transferred per redox centre. The reduction potential was determined to be identical for the CO dehydrogenase, semi-apo-(CO dehydrogenase), and CO dehydrogenase with carbonyl sulphide (COS) or cyanide bound. The electron-transferring efficiency of CO dehydrogenase was affected by two inhibitors, COS and cyanide, as indicated by a diminished analytic current.

Sequential resonance assignments of oxidized high-potential iron-sulfur protein from Chromatium vinosum

2D NMR spectra of the high-potential iron-sulfur protein (HiPIP) from Chromatium vinosum have been used to obtain partial resonance assignments for the oxidized paramagnetic redox state of the protein. Sequence-specific assignments were made using NOESY and COSY spectra in H2O and D2O of the following backbone segments: Asn-5-Arg-33, Glu-39-Asp-45, Gly-55-Cys-63, Gly-68-Ala-78, and Leu-82-Gly-85. NOESY spectra with a spectral width wide enough to include the hyperfine-shifted resonances revealed numerous NOE contacts between these signals and those in the main envelope of the proton spectrum. With the aid of the X-ray crystal structure [Carter, C.W., Kraut, J., Freer, S. T., Xuong, N. H., Alden, R. A., & Bartsch, R. G. (1974) J. Biol. Chem. 249, 4212], these NOEs permitted seven of the nine hyperfine-shifted signals to be assigned to three of the cysteine residues liganded to the metal cluster (Cys-43, Cys-46, and Cys-77). The other two hyperfine-shifted signals produced no detectable NOEs to other resonances in the spectrum and were tentatively assigned to the remaining cysteinyl ligand (Cys-63). These assignments, in conjunction with recent theoretical models of the electronic structure of the Fe4S4 cluster [Noodleman, L. (1988) Inorg. Chem. 27, 3677; Bertini, I., Briganti, F., Luchinat, C., Scozzafava, A., & Sola, M. (1991) J. Am. Chem. Soc. 113, 1237], indicate that the iron atoms coordinated to Cys-63 and Cys-77 are those of the mixed-valence Fe(3+)-Fe2+ pair whereas Cys-43 and Cys-46 are ligands to the Fe(3+)-Fe3+ metal pair.

A totally synthetic histidine-2 ferredoxin: thermal stability and redox properties

The entire polypeptide of Clostridium pasteurianum ferredoxin (Fd) with a site-substituted tyrosine-2----histidine-2 was synthesized using standard t-Boc procedures, reconstituted to the 2[4Fe-4S] holoprotein, and compared to synthetic C. pasteurianum and native Fds. Although histidine-2 is commonly found in thermostable clostridial Fds, the histidine-2 substitution into synthetic C. pasteurianum Fd did not significantly increase its thermostability. The reduction potential of synthetic histidine-2 Fd was -343 and -394 mV at pH 6.4 and 8.7, respectively, versus standard hydrogen electrode. Similarly, Clostridium thermosaccharolyticum Fd which naturally contains histidine-2 was previously determined to have a pH-dependent reduction potential [Smith, E.T., & Feinberg, B.A. (1990) J. Biol. Chem. 265, 14371-14376]. An electrostatic model was used to calculate the observed change in reduction potential with pH for a homologous ferredoxin with a known X-ray crystal structure containing a hypothetical histidine-2. In contrast, the reduction potential of both native C. pasteurianum Fd and synthetic Fd with the C. pasteurianum sequence was -400 mV versus standard hydrogen electrode and was pH-independent [Smith, E.T., Feinberg, B.A., Richards, J.H., & Tomich, J.M. (1991) J. Am. Chem. Soc. 113, 688-689]. On the basis of the above results, we conclude that the observed pH-dependent reduction potential for both synthetic and native ferredoxins that contain histidine-2 is attributable to the electrostatic interaction between histidine-2 and iron-sulfur cluster II which is approximately 6 A away.

Electrochemistry of Cu(I) bipyridyl complexes with alkene, alkyne, and nitrile ligands. Implications for plant hormone action of ethylene

The redox behavior was evaluated for several (BIPY)Cu(I) complexes (BIPY = 2,2'-bipyridyl) with unsaturated ligands by means of cyclic voltammetry in CH2Cl2 at reduced temperatures (-78 degrees, -23 degrees, 0 degree C). The complexes studied are [Cu(I)(BIPY)(C2H4)]PF6, [Cu(I)(BIPY)(3-hexyne)] PF6, [Cu(I)(BIPY)(DEAD)]PF6, ([Cu(I)(BIPY)]2 DEAD)[PF6]2 (DEAD = diethyl acetylene dicarboxylate) and [Cu(I)(BIPY)(CH3CN)]PF6. The oxidations are quasi-reversible at -78 degrees C for scan rates of 20 to 200 mV/sec. The reductions were irreversible on the CV time scale. Evidence is presented in support of a role for an electron transfer mechanism in the case of the plant hormone ethylene. Related literature data are also discussed.

Redox properties of several bacterial ferredoxins using square wave voltammetry

The equilibrium reduction potential of the 2[4Fe-4S] ferredoxin (Fd) isolated from four different bacterial strains was determined at a methyl viologen-modified gold electrode using square wave voltammetry. The observed reduction potential at pH 8 for Clostridium thermoaceticum Fd was -385 mV; Clostridium pasteurianum, -393 mV; Clostridium thermosaccharolyticum, -408 mV; and Chromatium vinosum, -460 mV versus normal hydrogen electrode at 25 degrees C. The reduction potential of the C. pasteurianum Fd was found to be pH independent from pH 6.4 to 8.7, indicating that the electron transfer mechanism does not involve proton exchange. In contrast, the reduction potential of the C. thermosaccharolyticum Fd was found to be pH dependent from pH 6.4 to 8.7, with pKox approximately 7 and pKred approximately 7.5. The +30 mV change in reduction potential from pH 8.7 to 6.4 was attributed to an electrostatic interaction between the iron-sulfur cluster II and the protonated histidine 2 residue located about 6 A away. The Ch. vinosum Fd interacted reversibly at the methyl viologen-modified gold electrode, and its reduction potential was verified using visible spectroelectrochemistry. The reduction potential of Ch. vinosum Fd was found to be 30 mV more positive than previously reported. The similarities of the bacterial Fd reduction potentials are discussed in terms of the homology of their primary structure as reflected by the similarities in the visible and circular dichroic spectra.

Characterization of the recombinant Clostridium pasteurianum ferredoxin and comparison of its properties with those of the native protein

Ferredoxins (Fds) constitute an important class of nonheme iron-sulfur proteins. One of the most studied Fds is the [8Fe-8S] Fd from Clostridium pasteurianum. The gene for this Fd has previously been cloned and sequenced. We report the expression of this Fd in Escherichia coli, and the characterization and comparison of this recombinant protein to the native Fd. We have found that the purified recombinant protein has the same enzymatic, redox, magnetic and electronic properties as the native Fd isolated from C. pasteurianum, which indicates that the two [4Fe-4S] clusters present in the Fd were correctly formed in E. coli.

Are reduction potentials of antifungal agents relevant to activity?

Cyclic voltammetry data were obtained for several categories of fungicidal agents including quinones (akrobomycin, podosporin A), iminium ions and precursors (pyridazines, 15-azahomosterol, griseofulvin-4'-oxime), and metal derivatives of chelators (pyridine-2-aldehyde thiosemicarbazones). The reductions usually occurred in the range of -0.7 to +0.3 V. Reduction potentials provide information on the feasibility of electron transfer in vivo. Catalytic production of oxidative stress from redox cycling is a possible mode of action. Alternatively, there may be interference with normal electron transport chains.

Reduction potentials of imine-substituted, biologically active pyridines: possible relation to activity

Cyclic voltammetry data were obtained for a number of biologically active compounds which incorporate imine substitution on the pyridine nucleus. The reductions in acid (iminium ion formation) were for the most part reversible, and in the range of -0.5 to -0.7V. The toxic effect of these drugs is thought to be caused by the generation of reactive oxygen radicals that arise via charge transfer, or by disruption of electron transport chains.

Spectroelectrochemical studies of the corrinoid/iron-sulfur protein involved in acetyl coenzyme A synthesis by Clostridium thermoaceticum

An 88-kDa corrinoid/iron-sulfur protein (C/Fe-SP) is the methyl carrier protein in the acetyl-CoA pathway of Clostridium thermoaceticum. In previous studies, it was found that this C/Fe-SP contains (5-methoxybenzimidazolyl)cobamide and a [4Fe-4S]2+/1+ center, both of which undergo redox cycling during catalysis, and that the benzimidazole base is uncoordinated to the cobalt (base off) in all three redox states, 3+, 2+, and 1+ [Ragsdale, S.W., Lindahl, P.A., & Münck, E. (1987) J. Biol. Chem. 262, 14289-14297]. In this paper, we have determined the midpoint reduction potentials for the metal centers in this C/Fe-SP by electron paramagnetic resonance and UV-visible spectroelectrochemical methods. The midpoint reduction potentials for the Co3+/2+ and the Co2+/1 couples of the corrinoid were found to be 300-350 and -504 mV (+/- 3 mV) in Tris-HCl at pH 7.6, respectively. We also removed the (5-methoxybenzimidazolyl)cobamide cofactor from the C/Fe-SP and determined that its Co3+/2+ reduction potential is 207 mV at pH 7.6. The midpoint potential for the [4Fe-4S]2+/1+ couple in the C/Fe-SP was determined to be -523 mV (+/- 5 mV). Removal of this cluster totally inactivates the protein; however, there is little effect of cluster removal on the midpoint potential of the Co2+/1+ couple. In addition, removal of the cobamide has an insignificant effect on the midpoint reduction potential of the [4Fe-4S] cluster. A 27-kDa corrinoid protein (CP) also was studied since it contains (5-methoxybenzimidazolyl)cobamide in the base-on form.(ABSTRACT TRUNCATED AT 250 WORDS)

A spectroelectrochemical cell designed for low temperature electron paramagnetic resonance titration of oxygen-sensitive proteins

In this paper we describe an anaerobic titrator made virtually from glass with a small amount of high vacuum epoxy mounted directly to a quartz EPR tube. A complete titration may be carried out with as little as 600 microliters of sample. This cell features the anaerobic manipulation of an electrochemically poised solution from an electrochemical pouch to an EPR tube. The cell uses a gold foil working electrode and Ag/AgCl reference and counter electrodes. The reference and counter electrodes are isolated from the sample by leached Vycor glass. In the work reported here, we used this cell to determine the equilibrium redox potential of methyl viologen in an EPR titration. With methyl viologen as an indicator we found that the cell has a residual oxygen level of 1.5 microM with a leak rate of 0.005 nmol/min. After moving the solution into the EPR tube, freezing, performing EPR, and thawing, the potential of the methyl viologen solution drifted only 2 mV. During the titration, the poised potentials were stable, drifting only 1 mV/min. Formal potentials as low as -630 mV in a vitamin B12-type protein have been determined with this cell (S. R. Harder, W.-P. Lu, B. A. Feinberg, and S. W. Ragsdale (1989) Biochemistry, in press).

Electrostatic effect upon association of reduced nicotinamide adenine dinucleotide and equine liver alcohol dehydrogenase

The rate of association of equine liver alcohol dehydrogenase and its coenzymes exhibits a large pH dependence with slower rates at basic pH and an observed kinetic pKa value of approximately 9-9.5. This pH dependence has been explained by invoking local active site electrostatic effects which result in repulsion of the negatively charged coenzyme and the ionized hydroxyl anion form of the zinc-bound water molecule. We have examined a simpler hypothesis, namely, that the pH dependence results from the electrostatic interaction of the coenzyme and the enzyme which changes from an attractive interaction of the negatively charged coenzyme and the positively charged enzyme to a repulsive interaction between the two negatively charged species at the isoelectric point for the enzyme (pH 8.7). We have tested this proposal by examining the ionic strength dependence of the association rate constant at various pH values. These data have been interpreted by using the Wherland-Gray equation, which we have shown can be applied to the kinetics of enzyme-coenzyme association. Our results indicate that the shielding of the buffer electrolyte changes from a negative to a positive value as the charge on the protein changes at the isoelectric point. This result is exactly that which is predicted for electrostatic effects that depend on the charge of the protein molecule and is not consistent with predictions based upon the local active site effects. At low ionic strength values of 10 mM or less, approximately 75% of the observed pH dependence results from the enzyme electrostatic effects; the remaining pH dependence may result from active site effects.(ABSTRACT TRUNCATED AT 250 WORDS)

Methionine-80-sulfoxide cytochrome c: preparation, purification and electron-transfer capabilities

In order to explore the electron-transferring properties of methionine-80-sulfoxide cytochrome c, the pure, chromatographically homogeneous methionine-80-sulfoxide cytochrome c was previously published procedure (Ivanetich, K.M., Bradshaw, J.J. and Kaminsky, L.S. (1976) Biochemistry 15, 1144-1153) was found to produce a mixture of products. In the pure derivative, visible spectroscopy indicates that the 695 nm band indicative of the Met-80-Fe coordination is missing, amino acid analysis indicates that only one methionine is modified to the sulfoxide, and the E0' is found to be 240 mV vs. N.H.E. For succinate cytochrome c reductase activity, the Km for modified cytochrome was about one-ninth that of the native protein, while the maximum turnover number of the reductase with the modified protein was only about 54% of that with native protein. In contrast, the activity with cytochrome oxidase measured polarographically using ascorbate and TMPD under two different buffer/pH conditions, gave Km values that were very similar for both the native and modified cytochromes c, but the maximum turnover numbers of the oxidase with the modified protein were less than 40% of native in either buffer. It is concluded that the Met-80-sulfoxide cytochrome c in the reduced form is able to maintain substantially its heme crevice structure and thus maintain Km values similar to those of native protein. However, the low maximum turnover numbers for oxidase activity with the modified protein in the reduced state indicate that electron transfer itself has been significantly decreased, probably because the parity of acid/base and electrostatic interactions of Met-80 sulfur with the Fe in the two redox states has been disrupted.

Energetics of beta-oxidation. Reduction potentials of general fatty acyl-CoA dehydrogenase, electron transfer flavoprotein, and fatty acyl-CoA substrates

We have determined reduction potentials for porcine mitochondrial general fatty acyl-CoA dehydrogenase (GAD) and electron transfer flavoprotein (ETF) using an anaerobic spectroelectrochemical titration method. Computer simulation techniques were used to analyze the absorbance data. Nernst plots of the simulated data gave E'0, 7.1, quinone/semiquinone = -0.014 V and E'0, 7.1, semiquinone/hydroquinone = -0.036 V for ETF and E'0, 7.1, quinone/semiquinone = -0.155 V and E'0, 7.1, semiquinone/hydroquinone = -0.122 V for GAD. Using these techniques we have also determined a conditional reduction potential of -0.156 V for the chromophore producing fatty acyl-CoA substrate beta-2-furylpropionyl-CoA. From this value and our previous determination of the equilibrium constant for the transhydrogenation reaction between beta-2-furylpropionyl-CoA and the oxidized substrate crotonyl-CoA (Keq = 10.4), we have determined a reduction potential of -0.126 V for the butyryl-CoA/crotonyl-CoA couple. In light of the structural similarity between butyryl-CoA and octanoyl-CoA, the optimal substrate for GAD, the reduction potential for octanoyl-CoA should be similar to that for butyryl-CoA; i.e. fatty acyl-CoA substrates and GAD are essentially isopotential. The ability of octanoyl-CoA to reduce GAD quantitatively (Keq = 9.0) poses a dilemma in light of the nearly equal reduction potentials. We postulate that the stable charge-transfer complex formed between enzyme and optimal product is significantly lower in energy than enzyme and product and thus is responsible for pulling the reaction toward completion.

Complications of organic solvents in synthetic drug application

Aziridinylbenzoquinone (AZQ), Acridinyl Anisidide (m-AMSA), Tenipsode Thenvlidene-Lignan-(VM-26), and PCNU are relatively new synthetic drugs believed to have a wide spectrum of antitumor activity. Preparation of these drugs for administration requires reconstitution with an organic solvent which decomposes the thermoplastic components of I.V. infusion devices and chemo-dispensing pins. The products of this decomposition may lead to cellular damage and complicate the infusion of these agents.

Comparative nuclear magnetic resonance studies of high potential iron-sulfur proteins from Chromatium vinosum and Rhodopseudomonas gelatinosa. Additional hyperfine shifted resonances and pH-dependent structural perturbations

Proton NMR spectra and their dependence on pH are reported for the oxidized and reduced forms of the high potential iron-sulfur proteins from Chromatium vinosum and Rhodopseudomonas gelatinosa. Spectra of the protein from both species are very similar in the regions occupied by the hyperfine shifted resonances of protons located near the [Fe4S4(S-Cys)4] cluster. The oxidized proteins exhibit three new resonances that had not been previously detected, one at very low field (about 100 ppm) and two at very high field (about -30 ppm). Since only downfield hyperfine shifted peaks have been observed in all other iron-sulfur proteins, the upfield resonances may be unique to high potential 4-Fe centers and originate from protons other than those on the cysteinyl ligands to the cluster. The pH dependencies of the chemical shifts of a large number of aromatic and hyperfine-shifted resonances indicate that the ionization state of histidine-42 exerts an influence on the electronic properties of the cluster despite its being located relatively far away. The oxidation state of the cluster also affects the ionization equilibrium of the histidine; pKa values of 6.7 and 7.3 are measured in the oxidized and reduced protein, respectively. These observations support a previous proposal (Nettesheim, D. G., Johnson, W. V., and Feinberg, B. A. (1980) Biochim. Biophys. Acta 593, 371-383) based on kinetic and visible spectroscopic evidence that the ionization state of histidine-42 affects the stability and oxidation rate of the reduced cluster.

Molecular interpretation of kinetic-ionic strength effects

A recent and important approach to investigating electron transfer mechanisms of redox proteins has been through kinetic-ionic strength studies. There is, however, significant controversy as to whether such studies (1) yield information regarding the charge (or location) of the electron transfer site or (2) more simply reflect the influence of net or overall protein charge on the electrostatic interactions. A critical analysis using different theoretical approaches is made of our recent work and of the bulk of the published non-physiological small molecule-protein and protein-protein kinetic ionic strength studies; it is concluded that (1) the approximated Bronsted-Debye-Huckel equation can not be used at all for protein redox reactions, (2) irrespective of the theoretical approaches discussed, such studies do not provide information regarding the charge of the electron transfer site, (3) it is the net charge of the reactants that control the electrostatic interactions, (4) both the equation derived by Wherland and Gray and the full Bronsted-Debye-Huckel equation provide reasonably good approximations of net protein charge, (5) pH changes quantitatively modulate net protein charge, and (6) thus, protein redox rates need to be electrostatically corrected if relevant interpretations of kinetic-ionic strength experiments are to be made.

High-potential iron-sulfur proteins and their possible site of electron transfer

The electron-transfer mechanism of the Fe4S4 high-potential iron-sulfur proteins (HiPIP's) was explored via a stopped-flow spectrophotometric kinetic study of the reduction of Chromatium vinosum and Rhodopseudomonas gelatinosa HiPIP's by both native and trinitrophenyllysine-13 horse cytochrome c. The influence of electrostatic effects was also effectively partitioned from the redox process per se. The corrected rates were 12.3 X 10(4) and 3.8 X 10(4) M-1 s-1 for native with C. vinosum and R. gelatinosa HiPIP, respectively, and 17.5 X 10(4) and 5.46 X 10(4) M-1 s-1 for TNP-cytochrome c with the two HiPIP's, respectively. The faster rates of TNP-cytochrome c with the HiPIP's are unexpected in terms of possible steric interaction since lysine-13 is at the top of the heme crevice. In understanding the somewhat faster rates of the TNP-cytochrome c over native cytochrome c it is possible that (1) TNP-cytochrome c reacts more quickly since modification of the lysine-13 residue destabilizes somewhat the heme crevice or (2) in light of the hydrophobic nature of the trinitrophenyl group and the X-ray crystallographic structure of HiPIP, the TNP group facilitates electron transfer by interacting with a hydrophobic region on the HiPIP molecular surface. The region about the S4 sulfur atom is the most exposed and accessible hydrophobic region on the HiPIP surface, in addition to being the point of closest approach of the S4 to the external environment.

The role of histidine-42 in the oxidation-reduction mechanism of Chromatium vinosum high potential iron-sulfur protein

The second order rate constants for the oxidation of high potential iron-sulfur protein (Hipip) of Chromatium vinosum by ferricyanide were determined as a function of ionic strength and pH. From the ionic strength results, calculations were done to correct the rate constant at each pH for the electrostatic interactions between Hipip and ferricyanide. The electrostatic corrections are necessary since the charge of the protein changes as a function of pH and can mask the ionization of mechanistically important amino acid residues. An apparent pKa congruent to 7 was obtained from electrostatically corrected rate-pH profile, indicating the possible participation of histidine. Perturbation difference spectroscopic studies of Hipip as a function of pH also gave apparent pKa values of 6.9 and 6.7 for the reduced and oxidized protein, respectively. That it was indeed His 42 (the only His in the polypeptide) that was responsible for the kinetic and spectroscopic pKa values was demonstrated by modification of His 42 of Hipip by the histidine selective reagent diethylpyrocarbonate. No modification of Tyr 19 could be detected. It is concluded that either deprotonation or modification of His 42 results in the destabilization of the reduced cluster and thus a faster rate of oxidation. This work provides the first experimental evidence of the 'squeeze effect' mechanism (Carter, C.W., Jr., Kraut, J., Freer, S.T. and Alden, R.A. (1974) J. Biol. Chem. 249, 6339--6346) in which the polypeptide directly modulates the stability of the iron-sulfur cluster.

Partial purification and characterization of a new intracellular beta-glucosidase of Trichoderma reesei

A new intracellular beta-glucosidase was isolated from Trichoderma reesei. It was sequentially purified by (NH4)2SO4 precipitation and chromatography and rechromatography on Sephadex G-150. The enzyme has a mol.wt. of 98 000, optimal activity at pH 6.5, pI 4.4 and Km values of 6.7 mM and 3.3 mM for sophorose and cellobiose respectively. Possible functions of the enzyme may be regulation of cellulase induction and/or to serve as a proenzyme.

Partitioning of electrostatic and conformational contributions in the redox reactions of modified cytochromes c

The reduction of acetylated, fully succinylated and dicarboxymethyl horse cytochromes c by the radicals CH3CH(OH), CO2.-, O2.-, and e-aq' and the oxidation of the reduced cytochrome c derivatives by Fe(CN)3-6 were studied using the pulse radiolysis technique. Many of the reactions were also examined as a function of ionic strength. By obtaining rate constants for the reactions of differently charged small molecules redox agents with the differently charged cytochrome c derivatives at both zero ionic strength and infinite ionic strength, electrostatic and conformational contributions to the electron transfer mechanism were effectively partioned from each other in some cases. In regard to cytochrome c electron transfer mechanism, the results, especially those for which conformational influences predominate, are supportive of the electron being transferred in the heme edge region.

Cyanide reactivity of cytochrome c derivatives

The kinetic rates and equilibrium association constants for cyanide binding have been measured for a series of cytochrome c derivatives as a probe of heme accessibility. The series included horse and yeast cytochromes iodinated at Tyr 67 and 74, horse cytochrome formylated at Trp 59 in both a low and high redox potential form, the Met 80 sulfoxide derivative of horse cytochrome and the N-acylisourea heme propionate derivative of tuna cytochrome. Native cytochromes c are well known to bind cyanide slowly in a reaction simply first order both in cytochrome and cyanide up to at least 100 mM in cyanide. The derivative demonstrate markedly different kinetics which indicate the following conclusions. (1) In spite of chemical modification at different loci, all the derivatives have highly similar reactivity, suggesting common ligation structures and mechanisms for reaction. (2) Compared to native cytochromes, reaction rates are 10-20 fold greater. This is in accord with a more accessible heme crevice, but not a completely opened crevice. For the completely opened case, rate increases are expected to be between three and five orders of magnitude. (3) Reaction rates are either independent of cyanide concentration (zero order) or show only slight variation. A mechanism which accounts for the data over four orders of magnitude in concentration postulates a protein conformation step, opening of the heme crevice, as the rate determining step. This conformation change has a limiting rate of 6 . 10(-2) s-1.

Lactoperoxidase-catalyzed iodination of horse cytochrome c:monoiodotyrosyl 74 cytochrome c

Iodination of horse cytochrome c with the lactoperoxidase-hydrogen peroxide-iodide system results initially in the formation of the monoiodotyrosyl 74 derivative. This singly modified protein was obtained in pure form by ion exchange chromatography and preparative column electrophoresis. It shows an intact 695 nm absorption band, the midpoint potential of the native protein, a nuclear magnetic resonance spectrum which indicates an undisturbed heme crevice structure, a normal reaction with antibodies directed against native horse cytochrome c, and circular dichroic spectra in which the only changes from those of the native protein can be ascribed to the spectral properties of iodotyrosine itself. This conformationally intact derivative reacts with the succinate-cytochrome c reductase and the cytochrome c oxidase systems of beef mitochondrial particle preparations indistinguishably from the unmodified protein, showing that the region including tyrosine 74 is not involved in these enzymic electron transfer functions of the protein. The circular dichroic spectra of this derivative indicate that the minima observed at 288 and 282 nm in the spectrum of native ferricytochrome c originate from tyrosyl residue 74.

Multiple low spin forms of the cytochrome c ferrihemochrome. EPR spectra of various eukaryotic and prokaryotic cytochromes c

1. Despite the same methionine-sulfur:heme-iron:imidazole-nitrogen hemochrome structure observed by x-ray crystallography in four of the seven c-type eukaryotic and prokaryotic cytochromes examined, and the occurrence of the characteristic 695 nm absorption band correlated with the presence of a methionine-sulfur:heme-iron axial ligand in all seven proteins, they fall into two distinct classes on the basis of their EPR and optical spectra. The horse, tuna, and bakers' yeast iso-1 cytochromes c have a predominant neutral pH EPR form with g1=3.06, g2=2.26, and g3=1.25, while the bakers' yeast iso-2 and Euglena cytochromes c, the Rhodospirillum rubrum cytochrome c2, and the Paracoccus denitrificans cytochrome c550 all have a predominant neutral pH EPR form with g1=3.2, g2=2.05, and g3=1.39. The ferricytochromes with g1=3.06 have a B-Q splitting that is approximately 150 cm-1 larger than the ferricytochromes with g1=3.2. 2. Each of the cytochromes displays up to four low spin EPR forms that are in pH-dependent equilibrium and can all be observed at near neutral pH. As the pH is raised the predominant neutral pH form is converted into two forms with g1=3.4 and g1=3.6, identified by comparsion with model compounds and other heme proteins as epsilon-amino:heme-iron:imidazole and bis-epsilon-amino:heme-iron ferrihemochromes, respectively. 3. The pK for the conversion of the predominant neutral pH EPR form into the alkaline pH forms is the same as the pK for the disappearance of the 695 nm absorption band for the cytochromes, even though these pK values range over 2 pH units. This confirms that the g1=3.06 and g1=3.2 forms contain the methionine-sulfur:heme-iron axial ligand while the g1=3.4 and the g1=3.6 forms do not. 4. At extremes of pH, the horse and bakers' yeast iso-1 proteins display several high and low spin forms that are identified, showing that a variety of protein-derived ligands will coordinate to the heme iron including methionine and cysteine sulfur, histidine imidazole, and lysine epsilon-amine. 5. The spectrum of horse cytochrome c with added azide, cyanide, hydroxide, or imidazole as axial ligands has also been examined. 6. From a comparison of the EPR and optical spectral characteristics of these groups of cytochromes with model compounds, it is suggested that the difference between them is due to a change in the hydrogen bonding or perhaps even in the protonation of N-1 of the heme iron-bound histidine imidazole.

Separate intramolecular pathways for reduction and oxidation of cytochrome c in electron transport chain reactions

The monoiodotyrosine 74, formyltryptophan 59, mononitrotyrosine 67, and carboxymethylmethionine 80 derivatives of horse cytochrome c are defective in their ability to accept electrons from the succinate-cytochrome c reductase system, while their reactions with purified cytochrome c oxidase are essentially those of the native protein. The 4-nitrobenzo-2-oxa-1,3-diazole derivative of lysine 13 of horse cytochrome c and the bis-phenylglyoxal derivative of arginine 13 of Candida krusei cytochrome c have the opposite properties, in that they are readily reduced by the succinate-cytochrome c reductase (EC 1.3.99.1) system but are defective in their capability of transferring electrons to cytochrome c oxidase (EC 1.9.3.1). We conclude that electrons from mitochondrial cytochrome c reductase are transmitted to ferricytochrome c by a different pathway than electrons from ferrocytochrome c to cytochrome c oxidase. The present results are compatible with the concept that the mechanism of reduction involves an aromatic ring channel comprising residues 74, 59, 67, and 80, leading from the "left back" part of the protein to the heme iron. On the other hand, since residue 13 is immediately above the edge of the heme that is at the "front surface" of the molecule, we suggest that the electron leaves ferrocytochrome c to cytochrome c oxidase by way of the edge of pyrrole ring II or the adjacent surface-located sulfur of cysteinyl residue 17, which is thioether bonded to the heme. On this basis, the sites of electron entry and exit in cytochrome c would appear to be some 110 degrees of arc away from each other along the surface of the protein, explaining several previously observed phenomena.