Myotonic dystrophy: investigation of the proposed defect in guanidoacetic acid synthesis.

No significant abnormality was detected in plasma levels or urinary excretion of guanidoacetic acid (GAA) in patients with myotonic dystrophy. It therefore seems unlikely that there is defective synthesis of GAA in this disorder. It was confirmed that arginine-glycine amidinotransferase (AGA), the enzyme responsible for GAA synthesis, is present in renal cortical tissue, but no enzyme activity could be detected in a variety of other, more accessible tissues taken from healthy controls.