複選題20. Which of the following statements is correct concerning biosynthesis of purine nucleotides? (A) Glycine, aspartate and glutamate are the nitrogen sources of the purine ring. (B) The major determinant of the rate of de novo purine nucleotide biosynthesis is the concentration of5-phosphoribosyl 1-pyrophosphate. (C) von Gierke disease reflects a defect in hypoxanthine-guanine phosphoribosyl transferase, an enzyme of purine salvage. (D) AMP and GMP also inhibit hypoxanthine-guanine phosphoribosyl transferase, which converts hypoxanthine and guanine to AMP and GMP. (E) All are true.