Nucleotide synthesis from purine bases and nucleosides in rat skeletal muscle.

An increasing amount of information on purine metabolism suggests that there is a continual release of some purines from resting skeletal muscle (Bockman et al., 1976). During exercise, hypoxanthine and inosine release can increase substantially (Murray, 1971). Presumably, skeletal muscle must be capable of maintaining an adequate level of purines in view of an apparent loss of purine nucleosides and bases. This has led us to examine the metabolic routes by which skeletal muscle can maintain its purine content. The ability of skeletal muscle to synthesize purine nucleotides de nouo has been demonstrated (Sheehan et al., 1977). The alternative mechanism for maintenance of purine nucleotide levels is by so-called ‘salvage’ of purine bases and nucleosides. The present investigation was directed towards determining the rate and extent of purine salvage in rat skeletal muscle. Synthesis of purine nucleotides was investigated by addition of labelled nucleosides or bases to incubated extensor digitorum longus muscles isolated intact from the rat hindlimb. Muscles were removed from the medium after 60min incubation, frozen in liquid N2 and the labelled nucleotides were separated and measured as previously described (Sheehan et a/., 1977). The results showed that skeletal muscle is capable of synthesizing purine nucleotides by the ‘salvage’ pathways. The rates of nucleotide formation (Table 1) compare favourably with those measured in other tissues (Wong & Henderson, 1972; Namm, 1973). As appreciable amounts (30-70%) of each of the radioactive substrates remained unused under the conditions of the experiments, it is unlikely that the level of the precursor was rate-limiting. Based on the amount of radioactivity in the individual bases, nucleosides and nucleotides and on what is known about purinemetabolism in general and that in skeletal muscle in particular, an attempt can be made to identify the preferred enzymic routes of nucleotide formation from each of the precursors used.