The case for assigning a value of approximately 4 to p K a I of the essential histidine-cysteine interactive systems of papain, bromelain and ficin

Protein electron transfer (mechanism and reproductive toxicity): iminium, hydrogen bonding, homoconjugation, amino acid side chains (redox and charged), and cell signaling

This contribution presents novel biochemical perspectives of protein electron transfer (ET) with focus on the iminium nature of the peptide link, along with relationships to reproductive toxicity. The favorable influence of hydrogen bonding on protein ET has been widely documented. Hydrogen bonding of the zwitterionic peptide enhances iminium character. A wide array of such bonding agents is available in vivo, with many reports on the peptide link itself. ET proceeds along the backbone, due in part, to homoconjugation. Redox amino acids (AAs), mainly tyrosine (Tyr), tryptophan (Typ), histidine (His), cysteine (Cys), disulfide, and methionine (Met), are involved in the competing processes for radical formation: direct hydrogen atom abstraction versus electron and proton loss. It appears that the radical or radical cation generated during the redox process is capable of interacting with n-electrons of the backbone. Beneficial effects of cationic AAs impact the conduction process. A relationship apparently exists involving cell signaling, protein conduction, and radicals or electrons. In addition, the link between protein ET and reproductive toxicity is examined. A key element is the role of reactive oxygen species (ROS) generated by protein ET. There is extensive evidence for involvement of ROS in generation of birth defects. The radical species arise in protein mainly by ET transformations by enzymes, as illustrated in the case of alcoholism.

Selective labeling of sulfhydryls and disulfides on blot transfers using avidin-biotin technology: studies on purified proteins and erythrocyte membranes

Various strategies for the use of 3-(N-maleimido-propionyl) biocytin (MPB) as a general label for distinguishing between protein sulfhydryls and disulfides on blot transfers are presented. In the first approach, endogenous SH groups in proteins were labeled directly with MPB. For disulfide staining, endogenous sulfhydryls were blocked with N-ethylmaleimide, disulfides were then reduced with mercaptoethanol, and the newly formed SH groups were labeled with MPB. In this approach, all treatments were performed in vitro, and, between steps, excess reagent was removed by dialysis. The MPB-labeled proteins were then separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) (in the presence of mercaptoethanol), the labeled proteins were transferred to nitrocellulose, and the blotted proteins were detected by avidin-biotin technology. In the second approach, MPB treatment was performed directly on blot transfers. For SH labeling, proteins were subjected to SDS-PAGE in the absence of mercaptoethanol, thus retaining the status of endogenous sulfhydryl and disulfide groups. For S-S labeling, proteins were treated with N-ethylmaleimide in vitro and then subjected to SDS-PAGE in the presence of mercaptoethanol, such that endogenous sulfhydryls were blocked and endogenous disulfides were converted to SH groups. Subsequent treatments and washings were performed on blots. In the third approach, immobilized proteins (i.e., in artificial systems or in natural systems such as membrane preparations or intact cells) were treated essentially as described in the first approach, except that washings were carried out by centrifugation. In vitro treatments were performed before SDS-PAGE (carried out in the presence of mercaptoethanol) and subsequent blot transfer. The relative merits of the three strategies are discussed.