The nature of the differential inhibition of polyphenylalanine synthesis in extracts from Artemia salina and rabbit reticulocytes by the Phytolacca americana protein

Immunotoxins containing single-chain ribosome-inactivating proteins

We have summarized what is currently known about the distribution, biological role, and the mechanism of action of the single chain ribosome-inactivating proteins and described the purification of one of them, gelonin, as an example. ITs have been made with several of these proteins and, depending upon the antibody used for conjugation, these immunoconjugates can show specific in vitro cytotoxicity which is similar to that shown by equivalent ITs prepared with ricin A chain. The most potent of these conjugates have shown antitumor efficacy in a variety of animal tumor models, including both syngeneic rodent tumors and xenografts in nude or immunosuppressed mice. An important point needs to be addressed, however, before concluding that ITs containing single chain toxins will be clinically useful. A major problem with this approach is that it is likely that both the antibody and the toxin components of these conjugates will be immunogenic. Both antitoxin and antixenogenic immunoglobulin responses have been shown to occur in animals after infusion of IT, although it has not yet been clearly demonstrated that such antibody responses adversely effect the pharmacokinetics or the efficacy of immunoconjugates. Thus, preliminary enthusiasm over the efficacy of these new reagents must be tempered with the knowledge that their use in the clinic may be limited by the host immune responses or other as yet undefined factors. The fact that there are many immunologically distinct single chain ribosome-inactivating proteins does suggest one way of evading the antitoxin response, by a sequential treatment with a panel of immunoconjugates, each containing a different single chain toxin.

Requirements for antiribosomal activity of pokeweed antiviral protein

It has been known for some time that pokeweed antiviral protein acts by enzymatically inhibiting protein synthesis on eucaryotic ribosome systems. The site of this action is known to be the ribosome itself. In this paper we show that the pokeweed antiviral protein reaction against ribosomes is a strong function of salt concentrations, where 160 mM K+ and 3 mM Mg2+ retards the reaction, while 20 mM K+ and 2 mM Mg2+ allows maximum reaction rate. It is also shown, however, that an unidentified protein in the postribosomal supernatant solution, together with ATP, allows the ribosome to be attacked even in the presence of high salt. Kinetic analysis of the antiviral protein reaction has been carried out under both sets of conditions, and reveals that the turnover number for the enzyme is about 300-400 mol/mol per min. in each case. The Km for ribosomes is 1 microM in the presence of low salt and 0.2 microM at higher salt in the presence of postribosomal supernatant factors plus ATP. The antiviral protein reaction is also shown to be pH dependent and is controlled by a residue with pKa value of approx. 7.0, apparently a histidine. Stoichiometric reaction of the enzyme with iodoacetamide results in a significant loss of antiribosomal activity.