Numbered enzymes include: 1, SSADH; 2, arginine-glycine amidinotransferase (AGAT); and 3, guanidinoacetate methyltransferase. The dashed line represents the probable conversion of accumulated GABA to guanidinobutyrate via the action of AGAT, as previously reported (Watanabe et al 1994). Abbreviations: SSA, succinate semialdehyde; GHB, gamma-hydroxybutyrate; GABA, gamma-aminobutyrate; Gly, glycine; Arg, arginine; Orn, ornithine. The structure of guanidinobutyrate is also shown.