Creatine and Creatinine

Creatine is present in muscle, brain and blood. It is particularly important in its phosphorylated form as an immediate store of high-energy phosphoryl bonds for the generation of ATP from ADP. The chemical energy for the first few seconds of muscle contraction are supplied by ATP generated from this source.



Biochemical defect

Clinical features


X conversion of phenylalanine to tyrosine

1 in 10 000 births mental retardation


X levels of cysteine and cystine levels of homocystine and methionine

Tall, thin body, subluxation of lens mental retardation


levels of homogentisic acid

Ochronosis (pigmented connective tissues), arthritis

Figure MT.15

Figure MT.15

Phosphorus Cycle

Figure MT.16 Urea cycle

Creatine is phosphorylated by creatine kinase. The serum levels of this enzyme are used as a marker of muscle damage following trauma or myocardial infarction. Creatine kinase activity may also be raised however simply as a result of violent exercise.

Creatinine is the anhydride of creatine and is formed as a metabolite for excretion in the urine. The 24 H urinary excretion of creatinine is relatively constant for any given individual, while renal tubular re-absorption is small. This means that the value for creatinine clearance can be used as an approximation to glomerular filtration rate.

Purines and Pyrimidines

These substances are ring-based structures that occur widely throughout the body. They form the base components in the ribonucleotides and deoxyribonucleotides, i.e. the 'alphabet' triplets that code RNA and DNA strands. Purines and pyrimidines occur throughout the tissues as carriers of high-energy phosphoryl bonds, e.g. adenosine triphosphate (ATP), guanosine triphosphate (GTP) and uridine triphosphate (UTP). These molecules are also ubiquitous as the functional parts of many co-factors.

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