Accessibility of the N-ethylmaleimide-unreactive sulfhydryl of human erythrocyte Band 3

Combinatorial peptidomics: a generic approach for protein expression profiling

Traditional approaches to protein profiling were built around the concept of investigating one protein at a time and have long since reached their limits of throughput. Here we present a completely new approach for comprehensive compositional analysis of complex protein mixtures, capable of overcoming the deficiencies of current proteomics techniques. The Combinatorial methodology utilises the peptidomics approach, in which protein samples are proteolytically digested using one or a combination of proteases prior to any assay being carried out. The second fundamental principle is the combinatorial depletion of the crude protein digest (i.e. of the peptide pool) by chemical crosslinking through amino acid side chains. Our approach relies on the chemical reactivities of the amino acids and therefore the amino acid content of the peptides (i.e. their information content) rather than their physical properties. Combinatorial peptidomics does not use affinity reagents and relies on neither chromatography nor electrophoretic separation techniques. It is the first generic methodology applicable to protein expression profiling, that is independent of the physical properties of proteins and does not require any prior knowledge of the proteins. Alternatively, a specific combinatorial strategy may be designed to analyse a particular known protein on the basis of that protein sequence alone or, in the absence of reliable protein sequence, even the predicted amino acid translation of an EST sequence. Combinatorial peptidomics is especially suitable for use with high throughput micro- and nano-fluidic platforms capable of running multiple depletion reactions in a single disposable chip.

Lysosomal sulphate transport is dependent upon sulphydryl groups

Using thiol blocking agents, we examined the role of sulphydryl groups for function of the lysosomal sulphate transport system. Monothiol binding reagents, p-hydroxymercuribenzoic acid (p-HMB) and p-chloromercuribenzene sulphonic acid (p-CMBS), dithiol binding reagents such as CuCl2, the alkylating agent, N-ethylmaleimide (NEM), and NADH all inhibited lysosomal sulphate transport. The inhibitory effects of NEM and Cu2+ were not additive, suggesting that they both act upon the same critical sulphydryl group(s). Unlike the case for NEM, the inhibitory effects of Cu2+ were reversed by the reducing agent, dithiothreitol. Exposure to NEM resulted in a seven-fold increase in Km to 867 microM versus a control value of 126 microM and a modest decrease in Vmax to 99 pmolperunit beta-hexosaminidase per 30 s versus a control value of 129 pmolperunit beta-hexosaminidase per 30 s. Similar although somewhat less dramatic results were obtained using Cu2+ with an increase of Km to 448 microM and a Vmax of 77 pmolperunit beta-hexosaminidase per 30 s. The sulphate transport activity of detergent solubilized lysosomal membranes could be bound to a p-chloromercuribenzoic acid (p-CMB)-Sepharose sulphydryl affinity resin and eluted with mercaptoethanol. Sulphydryl groups thus appear to play a role in sulphate transport through effects on substrate affinity. Sulphydryl-binding appears to be a strategy that may be useful for purification of the transporter.

Membrane transport of conjugated and unconjugated bile acids into hepatocytes is susceptible to SH-blocking reagents

The present study indicates that SH-groups are essential for the uptake of [3H]taurocholate and [14C]cholate into isolated rat hepatocytes. Several sulfhydryl-modifying reagents viz. p-chloromercuribenzenesulfonate (PCMBS), N-ethylmaleimide (NEM), dithio-bis(5-nitropyridine) (DTNP), bromosuccinimide and HgCl2 inhibited uptake of bile acids in a concentration-dependent manner. PCMBS was the most effective inhibitor in the uptake of taurocholate, while NEM is preferentially blocking the cholate uptake. PCMBS inhibited both the sodium- dependent and the sodium-independent bile acid uptake. Two different moieties of SH-groups seemed to be important for bile acid transport. One group was susceptible to DTNP and NEM, whereas PCMBS was able to block another type of SH-groups in addition. Cell viability was altered by SH-blockers, except by PCMBS. Efflux studies with 86Rb+ demonstrated that the transmembrane potential of hepatocytes was less effected by 100 microM PCMBS in contrast to 100 microM HgCl2. Efflux of tetra[3H]phenylphosphonium and of [3H]aflatoxin in PCMBS-treated hepatocytes documented membrane integrity during at least 10 min. PCMBS did not reduce cellular ATP levels significantly (minus 7%) nor did it markedly increase the amount of the Trypan-blue stained hepatocytes (plus 8.5%). The blocking effect of PCMBS was immediate and was completely reversed by the addition of 500 microM dithiothreitol (DTT), indicating a specific interaction with sulfhydryl-groups. This antagonizing effect of DTT depends on the concentration and exposure time of PCMBS. Six other thiols viz. 2-mercaptoethanol, 1,2-dimercaptoethane, 1,4-dimercaptobutane, 1,6-dimercaptohexane, L-cysteine and L-glutathione were less effective. The results suggest that free SH-groups on the outer surface of hepatocytes play an important role in the uptake process for conjugated and unconjugated bile acids.

The dissociation of peripheral proteins from erythrocyte membranes brought about by p-mercuribenzenesulfonate

The organic mercurial p-mercuribenzenesulfonate in 5 mM phosphate buffer (pH 8.0) solubilized ankyrin, bands 4.1 and 4.2, and glyceraldehyde-3-phosphate dehydrogenase from spectrin-depleted erythrocyte membranes. Glyceraldehyde-3-phosphate dehydrogenase was the protein most readily solubilized, being almost completely extracted by 0.5 mM reagent. The solubilization of ankyrin was similar to that of band 4.2, both showing maximal solubilization with 1.0 mM reagent. Band 4.1 was not appreciably solubilized below 2.5 mM p-mercuribenzenesulfonate. N-Ethylmaleimide did not itself solubilize proteins from ghosts or spectrin-depleted vesicles, and pretreatment at low temperature by 4 mM N-ethylmaleimide did not prevent subsequent solubilization by the mercurial. However, pretreatment at 37 degrees C with N-ethylmaleimide inhibited subsequent solubilization of ankyrin and band 4.2 by the mercurial and also resulted in the loss of binding of 1 mol mercurial per mol band 3. These data suggest that release of ankyrin and band 4.2 from the membrane by mercurial is linked to modification of band 3 by the reagent. After incubation of intact erythrocyte membranes with 0.1 M NaCl, treatment with p-mercuribenzenesulfonate selectively solubilized actin from the membranes. The resulting actin-depleted membranes did not vesiculate, but became spherical and lost their biconcave shape. Fragmentation was observed after subsequent removal of spectrin at low ionic strength.