Catalytic nature of tyrosine hydroxylase in bovine ocular tissues

We performed a comparative study on tyrosine hydroxylase (TH), a rate-limiting enzyme in catecholamine biosynthesis, in two ocular tissues, the retina and the iris-ciliary body. Immunoblotting analysis and gel filtration study suggested no significant difference in their subunit structure and their oligomeric form. The optimal pH of TH was 6.0 in retina and 6.3 in iris-ciliary body. However, at the physiological pH (7.25), the retinal TH had only 15% of the maximum activity while TH in the iris-ciliary body had 70% of the maximum activity. In hydroxylapatite chromatography, both extracts showed different elution profiles; the major TH activity in retina appeared earlier (200 mM, phosphate concentration) than that in iris-ciliary body (300 mM). When these enzymes were phosphorylated by catalytic subunit of cAMP-dependent protein kinase, most of the activity shifted to the later peak in both enzymes. Also, the activity of dephosphorylated TH in iris-ciliary body shifted to the earlier peaks. These results indicate that native TH in retina is basically less phosphorylated and thus exists in a less activated form than that in iris-ciliary body.