With respect to the degradation in vitro and in vivo of the G–L bond, γ-glutamylamine cyclotransferase, a kidney enzyme, was reported to cleave a G–L dipeptide, yielding free lysine and 5-oxoproline (synonym: pyroglutamate) [38]. Ordinary mammalian gastrointestinal digestive enzymes cleave proteins into amino acids but leave the G–L dipeptide uncleaved after ingestion of crosslinked proteins.