For the most common neurodegenerative disorders, there is no simple, obvious, and effective biomarker for disease identification. Plasma biomarkers of neurodegenerative disorders should play an important role in early detection and diagnosis of diseases. The catechol isoquinoline compounds are considered as possible neurotoxins involved in the pathogenesis of Parkinson’s disease (PD). These catechol isoquinolines and enzymes involved in their biosynthesis and metabolism may be endogenous factors in the pathogenesis of PD. As we know, there exists a salsolinol N-methyltransferase (SNMT) in human brain and lymphocyte. In normal postmortem human brain regions, frontal cortex, caudate, putamen, and thalamus, the SNMT activity was significantly higher than the control, and was well relevant to PD. In parkinsonian lymphocytes, the SNMT activity was much higher than in control patients. In addition, there was significant positive correlation between the SNMT activity in the lymphocyte and the level of N-methylsalsolinol in cerebrospinal fluid from the same untreated PD patients. Mass spectrometry–based biomarker discovery, undoubtedly, is still in its early stage of infancy. However, in light of the potential of this technology to provide deep coverage of the body fluid proteomes, it will certainly consolidate its role in development molecular medicine into clinical practice.