Creatine Case Study

Case Study

History

Learning Goals /
Concept Map


Creatine and Related Compounds

Structure

Amino Acids

Creatine in the Body

Equilibrium

Creatine-Creatinine Equilibrium

Creatinine Test for Kidney Function

Detection

Regulation and Ethics

Amine & Nitrile Chemistry

Laboratory Synthesis

Chemical Analysis

Creatine-Phosphocreatine Equilibrium

Uses & Side Effects


Creatine-Creatinine Equilibrium

Creatinine
After its discovery in urine in the mid-1880s[1], creatinine was believed to be related to creatine by both structure and function[2]. As described in CREATINE-PHOSPHOCREATINE EQUILIBRIUM , creatine is used by skeletal muscle to increase the amount of ATP available for use. Creatine is then metabolized in the muscle, where the end-product is creatinine, an internal anhydride of creatine[2,3]. Creatinine is then released into the blood and is eventually excreted by the kidneys by two pathways: filtration by the glomeruli (90-95%) and secretion by the distal tubule (5-10%)[4].

Creatine-Creatinine Equilibrium In Vitro
As we have seen, creatine (Cr) and creatinine (Crn) are structurally related.  Creatinine is formed when creatine cyclizes and loses a molecule of water; hence, creatinine is the cyclic, internal anhydride of creatine.

In solution, creatine and creatinine exist in equilibrium, but there is a strong pH dependence.  In acidic solution, formation of creatinine is strongly favored, and the reaction is irreversible.  In neutral or basic solution, however, the reaction is reversible, with appreciable amounts of both creatine and creatinine present in solution[5,6,7].  Furthermore, as we will see below, temperature also plays a role in the creatine-creatinine equilibrium.

Creatine-Creatinine Equilibrium In Vivo
Although the degradation of creatine into creatinine is reversible in the laboratory (in vitro) at neutral pH, this is not true under physiological conditions (in vivo).  Indeed, isotopic labeling studies with 15N have shown that in the body, once creatine is converted into creatinine, it does not convert back[3].  For this reason, creatinine is said to be the end product of creatine metabolism.  Once creatinine is produced in the body, the kidneys excrete it in the urine[3,4].

The amount of creatinine excreted is proportional to muscle mass and is relatively constant for each person in a given state of health.  As a result, urinary creatinine levels are sometimes used as a benchmark to verify the accuracy of 24-hour urine collections[2] and to compare the levels of other urinary components[8].  Furthermore, because creatinine is excreted by the kidneys, changes in creatinine levels (both in the blood and urine) may indicate kidney failure, as well as a variety of other conditions.  For more information, please see CREATININE TEST FOR KIDNEY FUNCTION.
 
Significance of Creatine-Creatinine Equilibrium in Determination of Kidney Function
As we have seen, creatinine is the final breakdown product of human creatine metabolism, and once creatinine has been formed in the body, it is not converted back into creatine.  However, under certain conditions outside of the body, the reaction between creatinine and creatine is reversible, and an equilibrium between the two compounds is established. This has important implications for the storage of urine samples containing creatinine.  Several considerations are important when storing urinary creatinine samples:  relative initial amounts of creatine and creatinine, pH, temperature, and length of storage time.  For example, if a urine sample containing equal amounts of creatine and creatinine (as might be observed in a healthy child or in an adult with muscle disease) were stored at acidic pH (3.5-4.0) at physiological temperature (37 °C) for a day, the amount of creatinine would increase by about 20% (more if the sample were stored longer).  By contrast, if a healthy man gave a urine sample containing little or no creatine and the sample were stored at pH 5.0 at 37 °C for a day, the amount of creatinine would decrease by about 5%[7].  Taking the in vitro equilibrium between creatine and creatinine into consideration and choosing appropriate storage conditions for urine samples is therefore critically important for ensuring the validity of urinary creatinine test results.
 

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[1] Williams, Melvin H.; Kreider, Richard B.; Branch; J. David. Creatine: The Power Supplement. Human Kinetics: Champaign, 1999; pp 6, 196-197.
[2] Peters, John P.; Van Slyke, Donald D. “Creatine and Creatinine,” in Quantitative Clinical Chemistry: Interpretations. The Williams & Wilkins Company: Baltimore, 1946; Vol. 1, Ch. 12, pp 897-920.
[3] Wyss, Markus; Kaddurah-Daouk, Rima. “Creatine and Creatinine Metabolism.” Physiol. Rev., 2000, 80(3), 1107-1213.
[4] Graves, John W. “Diagnosis and Management of Chronic Kidney Disease.” Mayo Clinic Proc., 2008, 83(9), 1064-1069.
[5] Edgar, Graham; Shiver, H.E. “The Equilibrium between Creatine and Creatinine, in Aqueous Solution. The effect of hydrogen ion.” J. Am. Chem. Soc., 1925, 47, 1179-1188.
[6] Cannan, Robert Keith; Shore; Agnes. “The Creatine-Creatinine Equilibrium. The Apparent Dissociation Constants of Creatine and Creatinine.” Biochem. J., 1928, 22(4), 920-929.
[7] Fuller, Nigel J.; Elia, Marinos. “Factors Influencing the Production of Creatinine: Implications for the Determination and Interpretation of Urinary Creatinine and Creatine in Man.” Clin. Chim. Acta, 1988, 175, 199-210.
[8] Smith-Palmer, Truis. “Separation Methods Applicable to Urinary Creatine and Creatinine.” J. Chrom. B., 2002, 718, 93-106.