Normetanephrine is an endogenous metabolite present in Cerebrospinal_Fluid that can be used for the research of pertension[1][2].
Endogenous metabolites is defined as those that are annotated by Kyoto Encyclopedia of Genes and Genomes as substrates or products of the ~1900 metabolic enzymes encoded in our genome. It is clear in the body of literature that there are documented toxic properties for many of these metabolites[1].
[1]. Lee N, et al. Endogenous toxic metabolites and implications in cancer therapy. Oncogene. 2020 Aug;39(35):5709-5720.
[2]. Foti A, et al. The relationships of free to conjugated normetanephrine in plasma and spinal fluid of pertensive patients. J Clin Endocrinol Metab. 1982 Jul;55(1):81-5.