Dr Richard Sabina
Research and Selected Publications

Research Interests

Figure 1. Adenylate metabolism in human tissues and cells. Net catabolism of ATP during periods of energy imbalance disrupts adenylate kinase equilibrium and leads to increased production of AMP. Competition between AMPD and cNT-I for available substrate forms the basis for regulation of these branchpoint reactions. Hypoxanthine is the major salvageable substrate in all tissues and cells, but the developmental loss of adenylosuccinate synthetase in erythrocytes blocks the resynthesis of AMP from both salvage and de novo sources of IMP.

AMP deaminase (AMPD) is a highly regulated enzyme that catalyzes a branchpoint reaction in the ATP catabolic pathway and competes with AMP-preferring cytosolic 5’-nucleotidase (cNT-I) for available substrate (Figure 1). This competition is best illustrated during periods of energy imbalance in different striated muscles, where catabolic flow proceeds primarily through AMPD in exercising skeletal muscle and through cNT-I in ischemic cardiac muscle. Mammalian species, including man, contain three AMPD genes that exhibit tissue-specific and developmental patterns of expression. Alignments across human AMPD amino acid sequences reveal conserved C-terminal and divergent N-terminal domains. Altered gene expression and enzyme regulation in different clinical situations can provide clues about the functional significance of AMPD. There are several situations where AMPD overexpression or hyperactivity is associated with negative clinical outcome. For example, erythrocyte AMPD (AMPD3 isoform E) activity is increased in oxidative stress and Tarui's disease (glycogen storage disease VII; familial phosphofructokinase deficiency), and in both cases promotes accelerated turnover of adenine nucleotides leading to hemolysis. In addition, increased AMPD2 and AMPD3 expression in Alzheimer’s disease brain is associated with elevated ammonia levels that may contribute to dementia in these individuals.

AMPD assumes an even greater role in the energy metabolism of erythrocytes, where the developmental loss of adenylosuccinate synthetase prevents the synthesis of AMP from both de novo and salvage synthesis precursors of IMP (Figure 1). Consequently, AMP deamination in erythrocytes results in an irreversible depletion of adenine nucleotides. Furthermore, any increase in AMPD activity hastens the loss of adenine nucleotides and can have profound effects on energy metabolism that lead to increased hemolysis. Currently, we are examining mechanisms of erythrocyte AMPD (isoform E) regulation and are focusing on:

• a pH-dependent protein-lipid interaction between intracellular membranes and isoform E as a mechanism for enzyme inhibition.
• a Ca2+-dependent protein-protein interaction between calmodulin and isoform E as a mechanism for enzyme activation.

We are characterizing these intermolecular interactions using a combination of biochemical, cellular, molecular and structural approaches.

Selected Publications

"Regulation of AMP deaminase by phosphoinositides" B. Sims, D.K. Mahnke-Zizelman, A.A. Profit, G.D. Prestwich, R.L. Sabina, and A.B. Theibert J. Biol. Chem. 274, 25701-25707 (1999)

"Towards an understanding of the functional significance of N-terminal domain divergence in human AMP deaminase isoforms." R.L. Sabina and D.K. Mahnke-Zizelman Pharmacol. Therapeut. 87, 279-283 (2000)

"Myoadenylate deaminase deficiency" R.L. Sabina and E.W. Holmes Metabolic and Molecular Bases of Inherited Disease C.R. Scriver, A.L. Beaudet, W.S. Sly and D. Valle, eds. McGraw-Hill Publishers, 8th Edition, Ch.110 274, 2627-2638 (2001)

"Localization of N-terminal sequences in human AMP deaminase isoforms that influence contractile protein binding." D.K. Mahnke-Zizelman and R.L. Sabina Biochem. Biophys. Res. Comm. 285(2), 489-495 (2001)

"N-terminal sequence and distal histidine residues are responsible for pH-regulated cytoplasmic membrane binding of human AMP deaminase isoform E." D.K. Mahnke-Zizelman, and R.L. Sabina J. Biol. Chem. 277, 42654-42662 (2002)

"Expression, purification, and inhibition of in vitro proteolysis of human AMPD2 (isoform L) recombinant enzymes." A.L. Haas, and R.L. Sabina Protein Express. Purif. 27, 293-303 (2003)

"N-terminal extensions of the human AMPD2 polypeptide influence ATP regulation of isoform L." A.L. Haas, and R.L. Sabina Biochem. Biophys. Res. Comm. 305, 421-427 (2003)

Last modified on: Wednesday, 24-Mar-2004 12:48:44 CST