Biochemical Genetics (/) 



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darkens on contact with air and turns from 

 light to dark brown and finally to black. 

 This characteristic persists throughout the 

 life of the individual. Affected persons are 

 not otherwise greatly inconvenienced, al- 

 though in middle or old age there is a greater 

 tendency to develop arthritis. Older affected 

 individuals may also show a bluish discolora- 

 tion of their nose, ears, and knuckles, so that 

 cartilage, tendons, and even bones can de- 

 velop this pigment. 



Family, pedigree, and population studies 

 reveal (1) that normal parents may have 

 affected children of either sex, (2) several 

 siblings from normal parents may be affected, 

 (3) affected children appear with a much 

 higher incidence when their normal parents 

 are related than when they are unrelated. 

 In view of these results, and the fact that the 

 blackening of the urine is expressed fully or 

 not expressed at all, it can be concluded that 

 affected individuals are homozygous for a 

 single pair of autosomally linked genes. 



The blackening is known to be due to the 

 oxidation of a substance present in urine 

 called alcapton, or homogentisic acid, whose 

 chemical description is 2,5-dihydroxyphenyl- 

 acelic acid (cf. Figure 21-3). The disease is 

 called alcaptonuria,^ and affected individuals 

 are alcaptonurics. It should be noted also 

 that several pedigrees have been found in 

 which apparently the same phenotype is 

 attributable to the action of a single dominant 

 gene. It is not surprising that the same bio- 

 chemical phenotype can be produced by two 

 different genotypes, for we have already found 

 this to be true in the failure of brown pigment 

 formation in Drosophila. Since dominant 

 alcaptonuria has not been studied extensively 

 biochemically, we shall confine our attention 

 henceforth to the recessive form of this 

 disease. 



Alcaptonuria is clearly an inborn error of 

 metabolism, and results in the daily excretion 



^ The account following is based upon the work of 

 A. E. Garrod and subsequent investigators. 



of several grams of alcapton. A study of the 

 biochemistry of alcaptonurics shows that no 

 other substance, among numerous others 

 tested for, appears in abnormal quantities in 

 the urine or blood, and that the reducing 

 properties of the urine can be attributed en- 

 tirely to the alcapton it contains. From these 

 results it would seem that we have traced the 

 pedigree of causes back to, or very close to, 

 the primary effect of the gene. The question 

 we may ask is whether the abnormal gene 

 manufactures alcapton, or affects organelles, 

 or modifies enzymes. 



If alcapton is a substance produced by the 

 abnormal gene, it should be absent in homo- 

 zygotes for the normal allele. Now, when 

 alcaptonurics are fed five grams of alcapton, 

 approximately this additional amount is 

 excreted in the urine. But when normal 

 individuals are fed the same quantity of al- 

 capton, no alcapton is found in the urine. 

 If, however, normal individuals are fed eight 

 grams of alcapton, some alcapton is found 

 in the urine. We can conclude from this that 

 normal people have the abihty to metabolize 

 alcapton to another form which does not 

 become pigmented upon exposure to air, but 

 that this ability has been lost, apparently 

 completely, by alcaptonurics. So, the ab- 

 normal gene does not produce its effect by 

 forming alcapton as a unique substance. 

 Apparently alcapton is a normal product of 

 metabolism which does not accumulate in 

 normal individuals because it is further 

 metabolized rapidly, but which does accumu- 

 late in alcaptonurics because of an organelle 

 or enzymatic defect. Clear evidence has been 

 obtained that it is an enzyme which has been 

 changed in the abnormal genotype. For it 

 has been found that the blood of alcaptonurics 

 is deficient in an enzyme, normally present, 

 which catalyzes the conversion of alcapton 

 by oxidation to a noncolor-producing sub- 

 stance, and this enzyme, homogentisate oxi- 

 dase, is in fact missing in the liver of the al- 

 captonuric. 



