Further studies on the acid-precipitable radioactive substance formed during perfusion of Langendorff rat hearts with [ 14 C]adenosine have shown that very brief (30 s) ischaemia causes a sudden rise (20–35%) in its level in the tissue which is followed by the steady fall we have previously described. Analysis of the products of alkaline hydrolysis of this compound shows that at least 96% of the radioactivity appears in the form of a mixture of 2′- and 3′-AMP as would be expected for RNA while its relatively high resistance to dilute alkali suggests that it is poly A. Subcellular localization studies indicate that radioactivity enters all compartments of the cell, with maximum label in the nucleus. However, a significant proportion is present in the mitochondria and may be poly A acting as the mitochondrial storage form of adenine nucleotides whose existence we have proposed.
Perfusion of Langendorff rat hearts with [ 14 C]adenosine yields an acid-insoluble, radioactive product whose concentration falls during ischaemia. The properties of the substance show that it is a polyribonucleotide. It is suggested that it may be mitochondrial poly A acting as a storage form of adenine nucleotides.
A comparative study of the operation of the purine nucleotide cycle and of the activity of adenylosuccinase in extracts of muscle from the two strains of dystrophic mouse shows that the cycle is defective in both cases in the conversion of adenylosuccinate to AMP. However, adenylosuccinase activity is only slightly reduced in the standard conditions for its direct assay.