Creatinine biosynthesis

Creatinine is formed in muscle from creatine phosphate by irreversible, nonenzymatic dehydration and loss of phosphate (Figure 31-6). The 24-hour urinary excretion of creatinine is proportionate to muscle mass. Glycine, arginine, and methionine all participate in creatine biosynthesis. Synthesis of creatine is completed by methylation of guanidoacetate by S-adenosylmethio-nine (Figure 31-6). 

Figure 20.24 Biosynthesis of creatine, creatinine phosphate, and creatine from arginine.

Pathway of creatine biosynthesis. In GMT deficiency, precursor guanidinoacetate accumulates and the synthesis of creatine and phosphocreatine is severely reduced. Creatinine, a nonmetabolizable end product that is excreted by the renal system, is also diminished. 

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