Tyrosine Hydroxylase, the Rate- Limiting Enzyme in Catecholamine Biosynthesis Could be an Index of Functionality in Pheochromocytoma Diagnosis

Aim: Excess release of catecholamines is a characteristic for pheochromocytomas. The rate of catecholamine synthesis is determined by tyrosine hydroxylase (TH) enzyme. Usually, TH is detected by immunohistochemistry. In our study, we evaluated this enzyme in plasma from pheochromocytoma subjects establishing correlations with some metabolites.    

Subjects and Methods:10 subjects (9 women /1man aged 40-72 years ) clinically suspected of pheochromocytoma, were biochemically investigated for free plasma normetanephrines (NMNp)/ metanephrines (MNp), plasma chromogranin A (CgA), plasma TH and IGF-1.Comparison of tumoral metabolites values was done with our lab normal range. Statistical analysis used multiple regression to evaluate relationship between TH and all 3 parameters: NMNp/MNp/CgA. 

Results: All subjects showed excessive plasma NMNp (median: 1434 pg/mL); in 7 cases we observed an over-secretion of MNp (median: 441pg/mL) and CgA was higher in 9 cases (median: 668 ng/mL). TH was identified in all plasma samples with a median of: 2.08 ng/mL. Higher values of TH were detected in cases with an over-excess of metabolites. Good correlations were established between NMNp/TH (0.51), MNp/TH(0.81) and NMNp/CgA (0.71).    

Conclusion: Metabolites in excess are well correlated with TH values proving a great rate of  catecholamine synthesis in some cases. We could affirm TH could be used as an index of functionality in pheochromocytoma diagnosis.