Infection of sympathetic and sensory neurones with herpes simplex virus does not elicit a shut-off of cellular protein synthesis: implications for viral latency and herpes vectors

Infection of non-neuronal cell types with herpes simplex virus type 1 (HSV-1) results in the degradation of host mRNA (Kwong & Frenkel 1987) and a shutoff in host protein synthesis (Roizman et al. 1965). This effect is mediated by a virion associated protein that is encoded by the viral vhs gene (Read & Frenkel 1983). This virion host shutoff (VHS) helps regulate viral gene expression and promotes efficient viral replication during the lytic cycle (Kwong & Frenkel 1987). Cultured sympathetic and sensory neurones, in contrast to primary rat fibroblasts, PC-12 cells, and Vero cells, showed no reduction in protein synthesis following infection with HSV-1. The resistance of neurones to VHS may be important in allowing establishment of a latent infection. In addition, this finding has a favourable impact on the idea of using HSV as a vector to deliver foreign genes into neurones.

The disulfide folding pathway of potato carboxypeptidase inhibitor

Potato carboxypeptidase inhibitor (PCI) contains 39 amino acids and three disulfides. Reduced and denatured PCI refolds spontaneously in vitro to regain its native structure. The folding pathway of a recombinant form of this protein has been elucidated by structural analysis and stop/go folding experiments of both acid and iodoacetate-trapped intermediates. The results reveal that folding of PCI proceeds through an initial stage of nonspecific disulfide formation (packing), followed by disulfide reshuffling (consolidation) of partially packed intermediates to acquire the native structure. The process of nonspecific packing involves a sequential flow of fully reduced PCI through 1- and 2-disulfide intermediates and leads to the formation of scrambled 3-disulfide species. All three classes of intermediates are highly heterogeneous. Disulfide reshuffling occurs at the final stage which refines and consolidates the scrambled species to acquire the native conformation. The efficiencies of disulfide formation and disulfide reshuffling can be selectively regulated by redox potential. Disulfide formation is promoted by cystine or oxidized glutathione, whereas disulfide reshuffling requires free thiols, such as cysteine, reduced glutathione, or beta-mercaptoethanol. Consolidation of scrambled species to form the native PCI represents the major rate-limiting step. When folding of PCI was carried out in the presence of cystine (2 mM) alone, more than 98% of the intermediates accumulate as the scrambled species after 1 min of folding. Furthermore, denaturant (5 M GdmCl or 8 M urea) mainly disrupts the final stage of PCI folding and exerts no apparent influence on the early stage of nonspecific packing. The compositions of 1- and 2-disulfide intermediates, according to their high performance liquid chromatography patterns, remain indistinguishable regardless of whether folding is performed in the absence or presence of denaturant.

Modification of cysteine residues with N-methyl iodoacetamide

Cysteine residues derivatized with N-methyl iodoacetamide (MIAA) can be analyzed by the Edman sequencing with a high degree of reliability. By HPLC, the phenylthiohydantoin (PTH) derivative of MIAA-modified cysteine eluted between dimethylphenylthiourea and PTH-Ala--a wide gap which is not occupied or interfered with by any other by-products or PTH amino acids. During extended Edman degradation, the recovery of PTH derivative of MIAA-modified Cys was as quantitative and reproducible as that of other stable PTH derivatives such as Ala, Val, and Leu.

Controlling the speed of hirudin folding

The folding of hirudin undergoes an initial stage of non-specific packing, followed by consolidation (re-organization) of partially packed intermediates to attain the native structure [Chatrenet and Chang (1993) J. Biol. Chem. 268, 20988-20996]. Non-specific packing leads to the formation of scrambled hirudins as folding intermediates. A systematic study was carried out to search for conditions which would selectively control and enhance the processes of packing and consolidation. It is demonstrated here that under optimized conditions, including the use of cystine/cysteine and protein disulphide isomerase, the folding of hirudin in vitro can be achieved quantitatively within 30 s.

Electrophysiological and pharmacological evidence for the role of the nucleus accumbens in cocaine self-administration in freely moving rats

The goal of this study was to clarify the role of the nucleus accumbens septi (NAS) in the expression of cocaine self-administration behaviors. Rats were trained in a continuous reinforcement schedule to press a lever to activate a pump that provided an intravenous injection of cocaine. Once the rats were trained, neuronal activity in the NAS was monitored during cocaine self-administration with chronic recording techniques using permanently implanted microwires. In the NAS, 19% of 181 neurons exhibited either increased or decreased firing rates seconds prior to lever pressing (termed "anticipatory responses"), and 48% had altered, predominantly decreased, firing rates for a few minutes after lever pressing ("postcocaine responses"). Two-thirds of the neurons with anticipatory responses had postcocaine responses. Neurons with either of these response patterns were localized histologically to both core and shell regions of the NAS, with no statistically significant differences in the proportion of response types in either area. Analysis of videotaped cocaine self-administration behaviors revealed that anticipatory responses were specifically associated either with the animal orienting toward and pressing the lever or only with movements directly related to pressing the lever. Anticipatory-like phasic spike activity was not observed during similar movements unrelated to lever pressing. In some animals, D1 (SCH 23390) or D2 (pimozide) receptor antagonists were injected systemically prior to or during self-administration sessions to assess the effects of dopamine receptor blockade on anticipatory and postcocaine responses. Each antagonist, given separately, often produced extinction of lever pressing. Both antagonists blocked the post-cocaine inhibitory response of neurons that had anticipatory responses. Neither antagonist modified anticipatory unit responses, nor did they affect postcocaine inhibitory responses in neurons that did not exhibit anticipatory responses. Taken together, these results suggest that the role of the NAS in cocaine self-administration may consist of two different mechanisms: (1) An initiation or trigger mechanism, as represented by the anticipatory neuronal responses, in which the NAS participates in triggering or mediating the goal-directed behaviors (e.g., lever pressing) that lead to the acquisition of the reinforcing agent (e.g., cocaine).(ABSTRACT TRUNCATED AT 400 WORDS)

Production of disulfide-linked hirudin dimer by in vitro folding

A simple process of in vitro folding has been developed for the preparation of hirudin dimer. A variant of recombinant hirudin with Asp33 replaced by Cys was expressed in yeast and isolated by HPLC. Crude Cys33-hirudin contains heterogeneous products that are made of one species of primary sequence. They were together reduced/denatured, and allowed to re-fold in the sodium bicarbonate buffer (pH 8.3) alone. Active, homogeneous Cys33-hirudin monomer folded spontaneously with a first order rate constant of 0.05 +/- 0.01 min-1, followed by the oxidation of two Cys33 to produce the pure dimer. The folding yield was 90%. On an equal weight basis, both Cys33-hirudin monomer and the dimer exhibit thrombin inhibitory activity comparable to that of wild-type hirudin. Due to the presence of an extra cysteine, the folding of active hirudin monomer (formation of three native disulfides) was accelerated by at least 12-fold.

The disulfide folding pathway of hirudin elucidated by stop/go folding experiments

The folding pathway of hirudin was analyzed by structural characterization and stop/go folding experiments of acid-trapped intermediates. The results show that the folding is initiated by a near-random packing, followed by the reorganization and fine adjustment of partially compact intermediates to attain the active molecule. The process of packing is observed as the unfolded hirudin flows sequentially via three groups of equilibrated intermediates, namely one-disulfide, two-disulfide, and three-disulfide (scrambled species) isomers. Nearly all possible disulfide species were found to exist along the pathway. Specific tertiary interactions then take effect at the final stage, refining and consolidating the loosely packed intermediates, in the presence of free thiols, to form the active hirudin. The rate of packing and reorganization can be selectively regulated by a number of external factors, and conditions can be chosen to allow the completion of folding process within 10 min or 10 h.

Comparative expression of endogenous retrotransposons in adult mouse mammary gland during normal development and differentiation

Amplified expression of the endogenous retrotransposons, intracisternal A particles (IAPs) and murine leukemia virus-related elements (MLVEs), along with decreased expression of VL30 elements frequently occurs during mouse mammary tumorigenesis. We have now analyzed the expression of these retroelements during the normal developmental and differentiation cycle of the mammary gland as found in virgin, pregnant, lactating, and postlactation adult female BALB/c mice. Retrotransposon expression was either unchanged or decreased during the progressive stages of the cycle compared to virgin tissue. Likewise, growth of mammary epithelial cells in primary culture had little or no effect on expression of IAPs, MLVEs and VL30 sequences. Thus, the dramatic changes involving these retrotransposons in many mouse mammary tumors appear unrelated to any normal state.

Rapamycin, a potential disease-modifying antiarthritic drug

Rapamycin (RAPA), a potent immunosuppressive agent that prevents organ graft rejection in animal models of transplantation, possesses a mechanism of action different than that of cyclosporin A and FK-506. In this study, the pharmacological activity of RAPA in a variety of immune and inflammatory models was assessed in order to define better its potential utility as an antiarthritic agent. RAPA inhibited T cell-mediated inflammation in mouse methylated bovine serum albumin-induced delayed-type hypersensitivity (ED40 = 4.7 mg/kg p.o.) and produced oral ED50 of 2.0 mg/kg against developing adjuvant arthritis in rats (3-day dosing schedule) and 9.5 mg/kg in established adjuvant arthritis in rats (daily dosing schedule). In both models of adjuvant arthritis, effects of RAPA were maintained even after cessation of drug dosing. In contrast, after discontinuation of cyclosporin A (5- and 10-mg/kg doses), disease activity returned. RAPA was also effective in another T cell-mediated model, experimental allergic encephalomyelitis (ED50 approximately 5 mg/kg p.o.). At higher doses, RAPA significantly inhibited carrageenan paw edema in rats, a model of acute inflammation (ED40, 56 mg/kg p.o.), without increasing serum corticosterone levels. In this model, doses approximately 10 to 20 times greater than active doses in T cell-mediated models were required. RAPA at 1 to 50 microM did not inhibit in vitro human synovial phospholipase A2 or 5-lipoxygenase and cyclo-oxygenase activity in the human blood leukocyte assay. The total profile of RAPA suggests that it may be effective in the treatment of rheumatoid arthritis, multiple sclerosis and other autoimmune diseases.

Sensitivity and specificity of transcranial Doppler ultrasonography in the detection of high-grade carotid stenosis

We evaluated the ability of transcranial Doppler ultrasonography (TCD) of the middle cerebral artery to detect internal carotid artery luminal area stenosis of 70% or greater by comparing TCD with cerebral angiography in 79 subjects, including 22 with normal cerebral arteriograms and 57 with 30% to 100% internal carotid artery stenosis on at least one side. Of six TCD measurements assessed, the flow acceleration provided the greatest sensitivity (82%) and specificity (73%) in distinguishing 70% to 100% carotid stenosis from lesser degrees of stenosis. The overall accuracy of the flow acceleration was 78%, and the positive predictive value was 79%. A flow acceleration of 351.6 cm/sec2 or less was useful for detecting high-grade stenosis, but there was difficulty in distinguishing unilateral from bilateral high-grade stenosis. TCD measurement of the flow acceleration allows for fairly reliable detection of high-grade stenosis and can thus serve as a useful indirect component of a carotid noninvasive battery.

Self-pumped phase conjugation and four-wave mixing in 0 degrees - and 45 degrees -cut n-type BaTiO(3):Co

Relatively fast self-pumped phase-conjugate and four-wave-mixing rise times are reported in n-type cobalt-doped barium titanate. With the crystal oriented in a 45 degrees cut as compared with the same crystal in a 0 degrees cut we find a factor of 3 decrease in the 0-90% rise time to 800 ms with 25-mW input power at 514.5 nm. Also, the self-pumped phase-conjugate reflectivity increases from 20% to 40%. We deduce that the phase conjugation is from internally seeded stimulated photorefractive backscattering. The four-wave-mixing rise time of the 45 degrees -cut crystal is 4 ms with a reflectivity of 48% when the pumping beams are derived from self-pumped phase conjugation that has an input power of 25 mW.

Antitumor agents. 126. Novel 4 beta-substituted anilino derivatives of 3',4'-O,O-didemethylpodophyllotoxin as potent inhibitors of human DNA topoisomerase II

A series of derivatives of 3',4'-O,O-didemethylpodophyllotoxin have been synthesized and evaluated for their inhibitor activity against neoplastic cell growth (KB) and against human DNA topoisomerase II as well as for their activity in causing cellular protein-linked DNA breakage. The results show that the compounds possessing a 4 beta-anilino moiety either unsubstituted or substituted at the para (F, COOCH3, COCH3, CN, CH2CN, NO2) or meta (OH) positions or with an ethylenedioxy moiety showed the same or greater activity than etoposide in causing cellular protein-linked DNA breakage and in inhibiting DNA topoisomerase II. However, compared to the corresponding 4'-O-demethyl analogues, the 3',4'-O,O-didemethyl compounds have a similar potency in inhibition of DNA topoisomerase II but are less active in causing cellular protein-linked DNA breakage. Complete correlation between the three biological activities--cytotoxicity, inhibition of DNA topoisomerase II, and induction of protein-linked DNA breakage--was also not observed. This supports the possibility that the biological determinants of action among these compounds may be different.

Selective increase in the right hemisphere transcranial Doppler velocity during a spatial task

Transcranial Doppler ultrasonography of the middle cerebral arteries was performed during two tasks: sorting of mah-jongg tiles and vibratory stimulation. These tasks selectively increase cerebral blood flow by positron emission tomography. The purpose of this study was to determine if analogous increases in cerebral blood flow velocity could be detected. We measured flow velocity during right hand manipulation followed by left hand manipulation, or vice versa, with resting studies in between. The average increase in the mean velocity, by paired t-test, was significant for the right middle cerebral artery with both left hand (p < .0005) and right hand (p < .005) tile sorting. For the left middle cerebral artery, there was an increase in the mean velocity with right hand (p < .005) but not for left hand sorting (p = .13). These findings support the importance of the right hemisphere in the performance of this type of spatial task. No significant flow velocity increase occurred during vibratory stimulation.

Phospholipase A2 enzymes: regulation and inhibition

The phospholipase A2 enzymes are important components of the cellular machinery that responds to inflammatory stimuli and maintains cell homeostasis by membrane remodelling. Their role as the rate-limiting step in the production of pro-inflammatory lipid mediators makes these enzymes an important therapeutic target for the treatment of inflammatory disorders. Keith Glaser and colleagues explain how the two major groups of phospholipase A2, the secretory and cytosolic forms, are very different both structurally and enzymatically. Understanding the relative contributions of these different forms of phospholipase A2 to physiological and pathological conditions requires greater insight into their cellular regulation and the development of selective inhibitors.

Identification of productive folding intermediates which account for the flow of protein folding pathway

Reduced hirudin N-terminal fragment (Hir1-27, 4 cysteines) refolds to form two-disulfide structures. The isomer containing the native disulfide prevailed as the predominant product. The disulfide folding pathway was elucidated by trapping the intermediates with acid (4% trifluoroacetic acid). All six possible one-disulfide intermediates were detected to exist in equilibrium with molar ratio of approximately 1:1:1:0.4:0.4:0.18. These intermediates were purified and structurally characterized. They were also allowed to resume the folding by reconstituting into alkaline buffer in order to evaluate the productivity of individual intermediate along the pathway. These results demonstrate that the most productive intermediate that specifies the pathway flow is neither a well populated species nor the one that contains the native disulfides.

Polyethylene Glycol Enhanced Protein Refolding

Previous studies on the refolding of recombinant bovine carbonic anhydrase B (CAB) indicated that polyethylene glycol (PEG) significantly enhanced the recovery of active protein by reducing aggregation. To further test the ability of PEG to enhance refolding, three recombinant human proteins, deoxyribonuclease (rhDNAse), tissue plasminogen activator (rhtPA), and interferon-gamma (rhIFN-gamma) were refolded in the presence of PEG (3350 MW). rhDNAse produced from CHO cells was denatured in 7.2 M urea and refolded by rapid dilution to 4.0 M urea and 0.20 mg/ml protein. When a final PEG to rhDNAse molar ratio of 5 to 1 (0.1 milligram PEG, 3350 MW) was used in the dilution buffer, refolding was improved by 30% to yield complete recovery of active protein. Impure E. coli derived inclusion body preparations of rhDNAse were solubilized in 8 M urea and refolded by dilution to 4 M urea and 0.10 mg/ml protein. Refolding with a dilution buffer which yielded a final PEG to rhDNAse molar ratio of 10 to 1 (0.1 milligram PEG, 3350 MW) resulted in a three-fold increase in the recovery of active protein. When PEG was used in the dilution buffer, aggregation of rhDNAse did not occur during refolding in either case. rhtPA produced from CHO cells was denatured in 5 M guanidine hydrochloride (GuHCl) and refolded by rapid dilution to 0.10 M GuHCl and 0.20 mg/ml protein.(ABSTRACT TRUNCATED AT 250 WORDS)

A modification of the in vivo mixed lymphocyte reaction and rapamycin's effect in this model

Rapamycin, a novel macrocyclic immunosuppressive agent, suppresses murine T cell activation in vitro by mechanisms distinct from cyclosporin A (CsA). This study was designed to examine rapamycin and CsA in the host vs graft popliteal lymph node (PLN) model, an in vivo system of T cell-dependent lymphocyte activation. The PLN procedure was modified by using irradiated CTLL-2 cells of C57BL/6 origin, instead of primary mouse splenocytes, as the allogeneic stimulus in C3H/HeN recipient mice. PLN cell proliferation was determined by [3H]-thymidine uptake. We found that the host lymphocyte proliferative response to CTLL-2 cells (H-2b) is greater than the response to mouse Balb/c splenocytes (H-2d). Rapamycin (ip or po) produced a dose-related inhibition of the in vivo mixed lymphocyte reaction. By contrast, the effects of CsA and FK-506 were not dose related within the same dose range (0.006-12 mg/kg). These data indicate that rapamycin is an effective immunosuppressive agent and confirm its ability to affect the allogeneic T cell response in vivo. Furthermore, the pharmacological data suggest that this PLN model utilizing irradiated CTLL-2 cells as an allogeneic stimulus provides a reproducible system to examine mixed lymphocyte reactions in vivo.

Rapamycin prevents the onset of insulin-dependent diabetes mellitus (IDDM) in NOD mice

The effect of the immunosuppressive agent rapamycin (RAPA) was assessed in the non-obese diabetic (NOD) mouse which is an autoimmune model of IDDM. RAPA was prepared in a vehicle of 8% cremophor EL/2% ethanol and investigated in two studies. NOD/MrK female mice (six per group, study no. 1; 10 per group, study no. 2) were dosed three times per week p.o. by gavage from 56 to 170 days of age (study no. 1) or from 64 to 176 days of age (study no. 2). Mice treated with RAPA at 0.6 mg/kg, 6 mg/kg, or 12 mg/kg maintained normal plasma glucose through 170 or 176 days of age with 10%, 0%, and 0% incidence of diabetes respectively. In contrast, naive, vehicle-treated, or RAPA 0.06 mg/kg-treated mice exhibited elevated plasma glucose and disease incidence typical for female NOD mice. Mice which became diabetic had elevated levels of beta-hydroxybutyrate, triglycerides and cholesterol. These plasma lipid concentrations were positively correlated with the duration of hyperglycaemia (r = 0.85, 0.87 and 0.84 respectively). Outside of its ability to prevent diabetes, RAPA itself did not affect the lipid profile of the mice. Intervention therapy with RAPA was ineffective at reversing the course of disease after IDDM onset under these experimental conditions. Finally, we report here that prophylactic treatment with RAPA was able to protect against IDDM development in some RAPA-treated mice 41 weeks after cessation of treatment. These data show that orally administered RAPA is effective in preventing onset of disease in the NOD mouse, a relevant model of autoimmune type I diabetes in man.

Characterization of camptothecin-resistant Chinese hamster lung cells

Three camptothecin-resistant sublines (V79r, IRS-1r and IRS-2r) of V79 cells and their irradiation-sensitive mutants, IRS-1 and IRS-2, were developed by stepwise, continuous exposure to camptothecin (CPT). The degree of resistance varied among these cells. Based on the biochemical characterizations of these resistant cell lines, the mechanisms which could be responsible for the resistance to CPT were proposed to be: (a) a decrease in the intracellular accumulation of CPT with or without alteration of DNA topoisomerase I, (b) a decrease in the amount of DNA topoisomerase I, or (c) a decrease in the sensitivity of DNA topoisomerase I to CPT. The resistant cells which exhibited down-regulation of DNA topoisomerase I were collaterally sensitive to etoposide (VP-16) and its analogue, 4'-demethy-4 beta-(4"-fluoroanilino)-4-desoxypodophyllotoxin, despite the fact that there were equal amounts of DNA topoisomerase II in the parental and in the resistant cell lines. Alternating the usage of CPT and VP-16 for the treatment of cancer is indicated.

Inhibition of in vitro immunoglobulin production by rapamycin

Like FK506, rapamycin, a structural analog of FK506, is a strong immunosuppressant. The immunosuppressive effect of Rapa in in vitro IgG, IgM, and IgA production by human lymphocytes was examined in this study. To inhibit spontaneous or pokeweed mitogen-stimulated production of Ig by human peripheral blood lymphocytes, about one thousandfold lower concentrations of Rapa (IC50 = 0.3 nM-2 nM) were required than of cyclosporine (IC50 = 0.3 microM-2 microM). T cells were the direct targets of Rapa, because preincubation of T cells with Rapa abolished the T cells helper effect to T-dependent Ig production. Rapa also had direct suppressive effect on B cells, since Rapa suppressed IgG production by pure B cells stimulated with IL2 and Staphylococcus aureus Cowan I. Kinetic studies measuring IgG production and cell proliferation revealed that Rapa acted at the activation stage of T and B cells. Exogenous IL2 substantially reversed the inhibitory effect of CsA but not that of Rapa in Ig production. This study is the first report on the strong suppressive effect of Rapa on human humoral immune response with a quantitative comparison with that of CsA. The underlying mechanisms are also explored. The results indicate the potential usefulness of this drug in treatment of presensitized transplantation patients, with whom cytotoxic Ab is a major obstacle to a successful transplantation.

Identification of a reactive lysyl residue (Lys103) of recombinant human interleukin-1 beta. Mechanism of its reactivity and implication of its functional role in receptor binding

Recombinant human interleukin-1 beta (h-IL-1 beta) was chemically modified with 4-(N,N-dimethylamino)-4'-isothiocyanatoazobenzene-2'-sulfonic acid (S-DABITC), a water-soluble color reagent specific for lysine labeling. Modified h-IL-1 beta was digested by lysyl endopeptidase. Peptides containing labeled lysines were detected at the visible wavelength (436 nm) and isolated by HPLC. The modification sites were eventually determined by sequence analysis. The results revealed that Lys103, Lys92, Lys93, and Lys94 of h-IL-1 beta reacted selectively with S-DABITC. A 1-h incubation with 1 mM S-DABITC at room temperature resulted in a quantitative modification of Lys103, 22% of Lys92, 27% of Lys93, and 18% of Lys94, respectively. This modification was accompanied by a 20-fold decrease of the protein's ability to bind to the receptor. Furthermore, a mutant of h-IL-1 beta (M9, Glu105 substituted by Lys) exhibits markedly impaired receptor binding, and the S-DABITC reactivity of its Lys103 was found to be reduced by 90%. These findings suggest that Lys103 of h-IL-1 beta might play an important role in the h-IL-1 beta/receptor interaction.

The effect of rapamycin on kidney function in the Sprague-Dawley rat

The effects of rapamycin (RAPA) on kidney function and histology were investigated in the Sprague-Dawley rat and compared with cyclosporine. Drugs were administered orally in a Cremophor-ethanol formulation for 14 days in two separate studies. RAPA, at 1 mg/kg, had no effect either functionally or histologically on the kidney. At 10 mg/kg, RAPA depressed the gain in body weight by 20% in the rat but had only minor functional disturbances on urine output, plasma creatinine, and creatinine clearance in the kidney. It did not induce any histomorphologic abnormalities. CsA, at 25 mg/kg, produced functional alterations in the kidney including elevated plasma creatinine and depressed clearance of creatinine as well as depressed body weight gain (17%). Histologically, CsA induced proximal tubule damage. These results demonstrate that RAPA (10 mg/kg) does not produce nephrotoxicity in the Sprague-Dawley rat at doses three times higher than its effective immunosuppressive doses established in the rat.

The folding of hirudin adopts a mechanism of trial and error

Reduced and denatured hirudin (65 amino acids and 3 disulfides) refolds in vitro to become an active molecule. The folding process adopts a mechanism of "trial and error" without predominant pathways. Throughout the entire folding process, the 6 cysteines were about equally involved in the disulfide shuffling. Among the first 20% of 3-disulfide species accumulated during the early phase of refolding, two-thirds were inactive and were reshuffled in the presence of thiol catalyst to regain correct disulfide pairing. When refolding was performed in the presence of strong denaturant (guanidinium chloride) without thiol catalyst, 8% of the active hirudin was obtained. This figure is close to the probability (6.7%) that would be expected from the random disulfide pairing of a molecule containing 6 sulfhydryl groups.