Absorption and Utilization of Amino Acids

Volume I

Mendel Friedman

• Publisher: CRC Press
• Year: 2019
• Print ISBN: 9781138596672
• eBook ISBN: 9780429946158
• Topic: Medicina
• Subtopic: Biochimica in medicina

Containing 45 papers written by outstanding international authors from 14 countries, this three-volume compendium brings together the elements needed to understand the factors which influence the utilization of amino acids. The wide-ranging topics include descriptions of metabolic pathways and mechanisms of the biological utilization of amino acids, as well as factors that influence amino acid bioavailability in enteral and parenteral nutrition. The use of amino acids to improve the quality and safety of the diet is presented. Also discussed are amino acid precursors of biogenic amines and the role of amino acids in atherosclerosis, cancer, and immunity. Scientists from many disciplines will benefit from this broad overview.

Chapter 1

DISPENSABLE, INDISPENSABLE, AND CONDITIONALLY INDISPENSABLE AMINO ACID RATIOS IN THE DIET

L. Preston Mercer, Steven J. Dodds, and Diane L. Smith

TABLE OF CONTENTS

I. Introduction

II. Saturation Kinetics Model

III. Proteins

IV. Amino Acids

V. AA/DAA Ratio

References

I. INTRODUCTION

The purpose of this chapter is to discuss the determination of reasonable dietary ratios of amino acids and the effect of those ratios on physiological responses. Dietary amino acids (AA) provide nitrogen, essential carbon skeletons and energy for synthesis of protein, purines, pyrimidines, biogenic amines and various other biological molecules. The AA may be divided into the nutritional classifications of indispensable (IAA), conditionally indispensable (CAA), and dispensable (DAA) based on absolute ability or relative rates of synthesis in vivo (Table 1). Diets can often be evaluated based on the relative concentrations of these three categories of AA and their abilities to stimulate different physiological responses, such as rate of growth, tissue AA concentrations, etc.

Many physiological responses in animals can be shown to be functions of dietary AA concentration (or any limiting dietary nutrient) when one analyzes the data using the Saturation Kinetics Model (SKM).^1,2 Use of the SKM allows one to characterize and optimize some measured performance characteristic (weight gain, food intake, etc.) in an animal, thereby providing a mechanism for estimating requirement levels and/or dietary allowances for any given nutrient.³

In this chapter, we will discuss current views and approaches to the formulation of the AA portion of diets and the possible application of the Saturation Kinetics Model to their improvement.

II. SATURATION KINETICS MODEL

The derivation of and the rationale for the SKM have been published.^{1, 2, 3} Briefly, the SKM is based on the concept that an organism is characterized by a sequence of homeostatically constrained steady states. Physiological responses are the result of a series of enzymatically mediated steps, one of which is rate limiting and displays saturation kinetics. The model is descriptive of a wide range of physiological responses and the model equation is continuous in its derivatives.

Model equation. The equation is as follows:

$\text{r}=\frac{\text{b}{\left(\text{K}.\text{5}\right)}^{\text{n}}+\text{R}\text{max}{\left(\text{I}\right)}^{\text{n}}}{{\left(\text{K}.\text{5}\right)}^{\text{n}}+{\left(\text{I}\right)}^{\text{n}}}$

(1)

where r = physiological response, I = dietary concentration or nutrient intake, Rmax = maximum theoretical response, K.5 = concentration or intake for 1/2 (Rmax + b), n = apparent kinetic order, and b = intercept on r axis.

Observed data pairs (I,r) are fitted by standard nonlinear curve fitting techniques and the four derived parameters are calculated (b, n, K.5 Rmax). These parameters can then be used to generate a theoretical response curve. Since Equation 1 is continuous in its derivatives, a slope curve can be generated using Equation 2.

$\text{slope}=\frac{{\text{nI}}^{\text{n}-1}\left(\text{R}\max -\text{b}\right){\left(\text{K}.\text{5}\right)}^{\text{n}}}{{\left[{\left(\text{K}.\text{5}\right)}^{\text{n}}+{\text{I}}^{\text{n}}\right]}^2}$

(2)

It is also possible to calculate a smooth efficiency curve using Equation 3.

$\text{efficiency}=\left(\text{r}-\text{b}\right)/\text{I}$

(3)

The equations for slope and efficiency can be solved for the intakes of maximum slope (Ims) and maximum efficiency (Ime). The intake of maximum slope gives the dietary concentration at which the measured response is most sensitive to changes in dietary nutrient concentration. The intake of maximum efficiency gives the dietary concentration at which the measured response is maximized for the minimum intake. The equations for Ims and Ime are

Table 1
NUTRITIONAL CLASSIFICATION OF AMINO ACIDS^a

Indispensable	Conditionally indispensable	Dispensable
Isoleucine		Glutamate
Leucine		Glutamine
Valine		Aspartate
Tryptophan		Asparagine
Threonine		Serine
Histidine		Proline
Methionine	Cysteine	Glycine
Phenylalanine	Tyrosine	Alanine
Lysine
	Arginine

^a List shown for most mammals.

$\text{Ims}=\text{K}.5{\left[\left(\text{n}-1\right)/\left(\text{n}+1\right)\right]}^{1/\text{n}}$

(4)

$\text{Ime}=\text{K}.5{\left(\text{n}-1\right)}^{1/\text{n}}$

(5)

The dietary nutrient concentration (or intake) required for maintenance (Iro) is given by Equation 6:

$\text{Iro}=\text{K}.5{(-\text{b}/\text{R}\mathrm{m...}}^{}$

Table of contents

• Cover
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• Chapter 1 Dispensable, Indispensable, and Conditionally Indispensable Amino Acid Ratios in the Diet
• Chapter 2 Subdividing Amino Acid Requirements into Portions for Maintenance and Growth
• Chapter 3 Efficiency of Utilization of Amino Acids
• Chapter 4 Regulation of Intestinal Amino Acid Transport
• Chapter 5 Measurement of Protein Synthesis in Animal Tissue In Vivo
• Chapter 6 Carbon-11 Labeled Amino Acid Analogs as Imaging Agents for In Vivo Study of Amino Acid Transport in Tumors and Organ Systems
• Chapter 7 Absorption and Metabolism of Amino Acids Studied In Vitro, In Vivo, and with Computer Simulations
• Chapter 8 Methionine, Homocysteine, Cysteine—Metabolic Interrelationships
• Chapter 9 Histidine-Methionine Metabolic Interrelationships
• Chapter 10 Taurine in Biology and Nutrition
• Chapter 11 Interactions among Leucine, Isoleucine, and Valine with Special Reference to the Branched-Chain Amino Acid Antagonism
• Index