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Introduction

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High-Yield Terms

  • Glucogenic: relating to amino acids whose catabolic products can be used for net glucose synthesis during gluconeogenesis

  • Ketogenic: relating to amino acids whose catabolic products enter the tricarboxylic acid (TCA) cycle but cannot be used for net glucose synthesis during gluconeogenesis

  • Essential amino acid: any amino acid that cannot be formed de novo or from other precursor compounds within mammalian cells

  • Glutamate dehydrogenase: critical enzyme catalyzing the interconversion of glutamate and α-ketoglutarate, serves as a gateway reaction between cellular energy demands and nitrogen homeostasis

  • Asparaginase: enzyme catalyzing the deamination of asparagine to aspartate, enzyme is used clinically as a chemotherapeutic treatment for certain leukemias

  • Cystathionine β-synthase: enzyme involved in cysteine biosynthesis, deficiencies are most common cause of homocystinurias

  • Glycine cleavage complex: also called glycine decarboxylase, enzyme-catabolizing glycine, defects in activity cause glycine encephalopathy

  • Phenylketonuria (PKU): a particular form of hyperphenylalaninemia resulting from defects in the phenylalanine hydroxylase gene

  • Branched-chain amino acid: consists of leucine, isoleucine, and lysine; defects in the catabolic enzyme branched-chain keto acid dehydrogenase (BCKD) result in Maple syrup urine disease (MSUD)

  • Alkaptonuria: first inherited error in metabolism to be characterized, benign disease caused by defects in the tyrosine-catabolizing enzyme homogentisate oxidase, urine of afflicted patients turns brown on exposure to air

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Amino Acid Uptake

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All tissues have some capability for synthesis of the nonessential amino acids, amino acid remodeling, and conversion of nonamino acid carbon skeletons into amino acids and other derivatives that contain nitrogen. However, many amino acids cannot be synthesized by mammalian cells nor derived from other precursors and so must be acquired from the diet. The process of protein digestion followed by peptide and amino acid uptake by the gut is described in detail in Chapter 43. Briefly, proteins are degraded to free amino acids, di- and tripeptides by gut and pancreatic peptidases. Intestinal uptake of amino acids involves several different Na+-dependent amino acid transporters present in the apical membrane of intestinal luminal brush border cells. These include acidic, basic, and neutral amino acid transporters, as well as a proline and glycine transporter. The inability to effectively absorb amino acids from the gut can lead to serious developmental and physiologic consequences, such as in the case of Hartnup disorder (Clinical Box 30-1).

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CLINICAL BOX 30-1: HARTNUP DISORDER

Hartnup disorder was first described in 1956 in the Hartnup family in London as a renal aminoaciduria of neutral amino acids associated with a pellagra-like skin rash and episodes of cerebellar ataxia. The disorder is caused by a defect in neutral amino acid transport in the apical brush border membranes of the small intestine and in kidney proximal tubules. The transporter is a member of the solute carrier family, specifically the SLC6A19 transporter. SLC6A19 is also known as the system B(0)–neutral amino acid transporter 1 (B[0]AT1). SLC6A19 is responsible for the transport of neutral amino acids in a Na+-dependent transport ...

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