The use of photoactivated hetero-bifunctional reagents to cross-link cytochrome c to proteins that bind it by electrostatic interactions

Syntheses with isotopically labelled carbon. Methyl iodide, formaldehyde and cyanide

Many of the uniquely labelled synthetic precursors currently employed in the design of sophisticated radiolabelled compounds have their origins in the field of hot atom chemistry. Particularly, the development during the past few years of automated, on-line synthetic procedures which combine the nuclear reaction, hot atom and classical chemistry, and rapid purification methods has allowed the incorporation of useful radionuclides into suitable compounds of chemical and biochemical interest. The application of isotopically labelled methyl iodide, formaldehyde, and cyanide anion as synthetic intermediates in research involving human physiology and nuclear medicine, as well as their contributions to other scientific methodology, is reviewed.

Chemical crosslinking of cell membranes

The complexity of cell membranes makes the resolution of their macromolecular topology one of the more challenging problems in modern molecular and cellular biochemistry. Despite the difficulties inherent in any such analysis, a surprisingly simple yet powerful approach exists that has consistently yielded valuable results. This method is chemical crosslinking, in which cell membranes are treated with crosslinking reagents (usually bifunctional) which produce covalent linkages between membrane components. The resultant complexes are usually then separated and identified by electrophoresis. This review is intended to provide a guide to the investigator who is unfamiliar with this approach. The overall strategy of crosslinking is discussed including selection of reagents, conditions to optimize crosslinking and the cleavage of crosslinked complexes to regenerate the original target for identification purposes. The crosslinking of biological membranes is then reviewed with special emphasis on recent advances including macromolecular photoaffinity labeling, kinetic analysis to probe symmetry properties and potential artifacts that may complicate interpretation of results. Examples of specific applications of crosslinking to membranes are presented in tabular form. The final portion of the review discusses the synthesis and properties of the most widely employed crosslinking reagents. Available reagents are summarized in a series of comprehensive tables. It is hoped that our discussion will provide the uninitiated investigator with sufficient information to ascertain the applicability of chemical crosslinking to particular areas of interest.

Purification and properties of a cross-linked complex between cytochrome c and cytochrome c peroxidase

Cytochrome c (horse heart) was covalently linked to yeast cytochrome c peroxidase by using the cleavable bifunctional reagent dithiobis-succinimidyl propionate in 5 mM-sodium phosphate buffer, pH 7.0. A cross-linked complex of molecular weight 48 000 was purified in approx. 10% yield from the reaction mixture, which contained 1 mol of cytochrome c and 1 mol of cytochrome c peroxidase/mol. Of the total 40 lysine residues, four to six were blocked by the cross-linking agent. Dithiobis-succinimidylpropionate can also cross-link cytochrome c to ovalbumin, but cytochrome c peroxidase is the preferred partner for cytochrome c in a mixture of the three proteins. The cytochrome c cross-linked to the peroxidase can be rapidly reduced by free cytochrome c-557 from Crithidia oncopelti, and the equilibrium obtained can be used to calculate a mid-point oxidation-reduction potential for the cross-linked cytochrome of 243 mV. Mitochondrial NADH-cytochrome c reductase will reduce the bound cytochrome only very slowly, but the rate of reduction by ascorbate at high ionic strength approaches that for free cytochrome c. Bound cytochrome c reduced by ascorbate can be re-oxidized within 10s by the associated peroxidase in the presence of equimolar H2O2. In the standard peroxidase assay the cross-linked complex shows 40% of the activity of the free peroxidase. Thus the intrinsic ability of each partner in the complex to take part in electron transfer is retained, but the stable association of the two proteins affects access of reductants.