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Creatine analogue beta-guanidinopropionic acid alters skeletal muscle AMP deaminase activity.

Dietary supplementation of the creatine analogue beta-guanidinopropionic acid (beta-GPA) decreases in vitro skeletal muscle AMP deaminase (AMP-D) activity in rats. Downregulation of AMP-D activity was progressive and greater in fast-twitch muscles (70-80%) than in the slow-twitch soleus muscle (approximately 50%). The loss in AMP-D activity had little effect on inosine 5'-monophosphate accumulation in mixed-fiber muscle with intense tetanic contractions. In contrast, inosine 5'-monophosphate formation was evident earlier in fast-twitch red and white fiber sections of creatine-depleted animals during intense twitch contractions, indicating that fast-twitch muscle of beta-GPA-treated rats buffers decreases in the ATP/ADPfree ratio via deamination, even though AMP-D activity is less. Isoforms of skeletal muscle AMP-D mRNAs in mixed-fiber muscle were not altered by feeding beta-GPA for up to 9 wk. Creatine depletion did not alter total immunoreactivity; however, a redistribution of AMP-D immunoreactivity from primarily an approximately 80-kDa form toward lower apparent molecular mass species (approximately 60 and approximately 56 kDa) was observed. Posttranslational changes in AMP-D appear related to changes in activity.