The cytotoxin α‐sarcin behaves as a cyclizing ribonuclease

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Abstract
The hydrolysis of adenylyl(3′→5′)adenosine (ApA) and guanylyl(3′→5′)adenosine (GpA) dinucleotides by the cytotoxic protein α-sarcin has been studied. Quantitative analysis of the reaction has been performed through reverse-phase chromatographic (HPLC) separation of the resulting products. The hydrolysis of the 3′-5′ phosphodiester bond of these substrates yields the 2′-3′ cyclic mononucleotide; this intermediate is converted into the corresponding 3′-monophosphate derivative as the final product of the reaction. The values of the apparent Michaelis constant (K M), k cat and k cat/K M have also been calculated. The obtained results fit into a two-step mechanism for the enzymatic activity of α-sarcin and allow to consider this protein as a cyclizing RNase.