Purine Salvage begins with the free nitrogenous bases, hypoxanthine and guanine. Guanine combines with PRPP to form GMP, whereas Hypoxanthine combines with PRPP to form IMP. IMP can then be interconverted with AMP. So put another way it allows us to salvage AMP from hypoxanthine. Both reactions are catalyzed by HGPRT, which is the most important enzyme in this pathway. A less common purine salvage pathway starts with adenine, which is typically not produced by purine degradation--hence why it is lower yield. Adenine combines with PRPP to form AMP, in a reaction catalyzed by APRT.