Coordinate Regulation of Tetrahydrobiopterin Turnover and Phenylalanine Hydroxylase Activity in Rat Liver Cells

This work had two purposes: (i) to determine in vivo whether liver phenylalanine hydroxylase (PAH) is regulated by its substrates phenylalanine and tetrahydrobiopterin (BH4) as studies with purified enzyme suggest and (ii) to investigate in vivo the relationship between PAH activity and BH4turnover....

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Bibliographic Details
Published in:Proceedings of the National Academy of Sciences - PNAS 1995-01, Vol.92 (3), p.885-889
Main Authors: Mitnaul, Lyndon J., Shiman, Ross
Format: Article
Language:English
Subjects:
Animals
Binding, Competitive
Biochemistry
Biopterins - analogs & derivatives
Biopterins - metabolism
Cells, Cultured
Cultured cells
DNA
Enzyme Activation
Enzymes
Hepatocytes
Kinetics
Liver
Liver - cytology
Liver - enzymology
Phenylalanine - metabolism
Phenylalanine Hydroxylase - metabolism
Physiological regulation
Pterins
Pterins - metabolism
Radiocarbon
Rats
Rodents
Water - analysis
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Summary:This work had two purposes: (i) to determine in vivo whether liver phenylalanine hydroxylase (PAH) is regulated by its substrates phenylalanine and tetrahydrobiopterin (BH4) as studies with purified enzyme suggest and (ii) to investigate in vivo the relationship between PAH activity and BH4turnover. We found there are two BH_4 pools in hepatocytes, one that is metabolically available (free BH4) and one that is not (bound BH4). Bound BH_4 appears bound to PAH; the PAH-BH4complex has much less catalytic activity and is less readily phenylalanine activated than uncomplexed enzyme. Interconversion of activated and unactivated PAH and bound and free BH4is driven by phenylalanine; and free BH4concentration is determined by the state of activation and activity of PAH. In hepatocytes, BH4and PAH (subunit) concentrations are equal, all intracellular BH4appears to be available to PAH, and free BH4turns over rapidly (t(1/2} ≈ 1 hr). There is no evidence for feedback inhibition of BH4synthesis; the BH4synthetic rate appears high when free BH4concentration is high and low when free BH4is low. The data provide support in vivo that phenylalanine and BH4are positive and negative regulators of the activity and activation state of PAH in the proposed manner; they also imply that regulation of BH4turnover and PAH activity are linked processes, which are both controlled by phenylalanine concentration.
ISSN:0027-8424
1091-6490
DOI:10.1073/pnas.92.3.885