Conjugated normetanephrine in human and rat plasma and erythrocytes.

Enzymatic hydrolysis was applied to the deconjugation of normetanephrine (NMN) in plasma and red blood cell lysate. By this procedure, in human plasma 77% of total NMN circulated in sulfate-conjugated form, while in rat plasma 63% was in glucuronidated form. Total NMN in human lysate was significantly higher than in plasma (P less than 0.001) and was mostly in the free form, indicating that red blood cells may play an important role in metabolism of norepinephrine. Enzymatic hydrolysis is superior to the standard method by acid hydrolysis plus heat since: (1) more conjugated NMN is hydrolyzed in human plasma and (2) a smaller sample is needed for hydrolysis.