Cell preparation, protein release from

Experiments show that it is on the ribosomes that protein synthesis actually occurs. If cells synthesizing protein from radioactive amino acids are studied, the radioactivity is first found bound to the ribosomes, and is only later released from them as soluble protein. A cell suspension from which ribosomes have been removed can never be made to synthesize protein, whilst if they are subsequently replaced synthesis can proceed rapidly. But the ribosomes alone are inadequate. In order to incorporate radioactive amino acids into new protein there needs to be added to the ribosomes a preparation of soluble cell material which contains certain enzymes, the soluble low-molecular-weight messenger RNA (m-RNA), transfer RNA (t-RNA), ATP, GTP, and ions like magnesium and potassium. So what role do these various substances perform in protein synthesis ... 

The inhibitory effect of ricin on protein synthesis in cell-free systems is greatly increased in the presence of / -mercaptoethanol. This is because free A chain is the enzymatically active toxin, whereas A chain linked to B chain in whole ricin is not active [ 121]. In the absence of reducing agents, even high concentrations of intact toxin do not inactivate ribosomes. Presumably the catalytic site on the A chain is formed or exposed only when the A chain is released from the B chain. Free A chain, however, is non-toxic to intact cells since, in the absence of B chain, it lacks the ability to bind to and enter cells. Ricin A-chain preparations frequently show some level of toxicity, however, because the complete removal of contaminating B chain can be difficult to achieve [ 122]. 

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