Biochemistry, 4th Edition P87. Continuing Garrett and Grisham's innovative conceptual and organizing framework, "Essential Questions," BIOCHEMISTRY guides students through course concepts in a way that reveals the beauty and usefulness of biochemistry in the everyday world. Streamlined for increased clarity and readability, this edition also includes new photos and illustrations that show the subject matter consistently throughout the text. New end-of-chapter problems, MCAT practice questions, and the unparalleled text/media integration with the power of CengageNOW round out this exceptional package, giving you the tools you need to both master course concepts and develop critical problem-solving skills you can draw upon. | How Are Purines Degraded 823 The Purine Nucleoside Cycle in Skeletal Muscle Serves as an Anaplerotic Pathway Deamination of AMP to IMP by AMP deaminase Figure followed by resynthesis of AMP from IMP by the de novo purine pathway enzymes adenylosuccinate synthetase and adenylosuccinate lyase constitutes a purine nucleoside cycle Figure . This cycle has the net effect of converting aspartate to fumarate plus NH4 . Although this cycle might seem like senseless energy consumption it plays an important role in energy metabolism in skeletal muscle The fumarate that it generates replenishes the levels of citric acid cycle intermediates lost in amphibolic side reactions see Chapter 19 . Skeletal muscle lacks the usual complement of anaplerotic enzymes and relies on enhanced levels of AMP deaminase adenylosuccinate synthetase and adenylosuccinate lyase to compensate. Xanthine Oxidase Xanthine oxidase Figure is present in large amounts in liver intestinal mucosa and milk. It oxidizes hypoxanthine to xanthine and xanthine to uric acid. Xanthine oxidase is a rather indiscriminate enzyme using molecular oxygen to oxidize a wide variety of purines pteridines and aldehydes producing H2O2 as a product. Xanthine oxidase possesses FAD nonheme Fe-S centers and molybdenum cofactor a molybdenum-containing pterin complex see Figure as electrontransferring prosthetic groups. Its mechanism of action is diagrammed in Figure . In humans and other primates uric acid is the end product of purine catabolism and is excreted in the urine. Birds terrestrial reptiles and many insects also excrete uric acid but in these organisms uric acid represents the major nitrogen HUMAN BIOCHEMISTRY Severe Combined Immunodeficiency Syndrome A Lack of Adenosine Deaminase Is One Cause of This Inherited Disease Severe combined immunodeficiency syndrome or SCID is a group of related inherited disorders characterized by the lack of an immune response to infectious disease. This .
