506 - Heredity and Evolution 



delects has provided considerable support to 

 ihe "one gene, one enzyme" hypothesis (p. 

 530) as to the mechanism of genie action 

 (Chap. 27). 



The missing or defective enzymes responsi- 

 ble for phenylketonuria, alkaptonuria, and 

 albinism are indicated in Figure 26-26. Phen- 

 ylketonuria, in which a high concentration of 

 phenylpyruvic acid (Fig. 26-26) accumulates 

 in the blood and urine, is an exceedingly 

 serious condition. Always it is accompanied 

 by a grave degree of mental deficiency. The 

 condition can be partially controlled, how- 

 ever, by careful regulation of the phenylala- 

 nine intake of the patient. This is difficult, 

 however, because phenylalanine is one of the 

 essential amino acids (p. 137) that cannot be 

 eliminated from the diet completely and be- 

 cause phenylalanine is a constituent of al- 

 most every protein food. Alkaptonuria and 

 albinism, on the other hand, are not so seri- 



ous. The urine of an alkaptonuric individual 

 turns black upon exposure to air, because 

 homogentisic acid, the substance that accu- 

 mulates in the blood and urine, spontane- 

 ously undergoes oxidation into a blackish 

 end product (Fig. 26-26). An albino is over- 

 sensitive to sunburn and glare. Melanin (p. 

 530), the principal light-absorbing pigment 

 of the skin, hair, and eyes, cannot be manu- 

 factured in the absence of the proper en- 

 zyme. (Fig. 26-26). 



The rare infant who suffers from galacto- 

 semia cannot metabolize galactose, which is 

 absorbed, of course, from milk (p. 80). Ac- 

 cordingly, this sugar accumulates to toxic 

 levels in the body fluids, although consider- 

 able amounts are excreted via the urine. 

 Specifically, the missing enzyme is galac- 

 tosephosphate transferase. This enzyme is 

 necessary for the conversion of the milk 

 sugar-phosphate to glucose-phosphate, as was 



NH, 



CH 2 -CH-COOH 

 PHENYLALANINE 



blocked 



(no enzyme) 



rffH > 



PHENYLKETONURIA 



NH, 



tyrosine 



blocked 



M il l > 



ALBINISM 



-> MELANIN 



blocked 



CH 2 CO'COOH 



PHENYLPYRUVIC ACID 

 (in blood and urine) 



HO T\ 



ch 2 -cooh 

 homogentisic acid 



blocked 

 ALKAPTONURIA " 



^ 



intermediates, 

 -* on route to 

 Krebs Cycle 



Fig. 26-26. Enzymes missing in certain heritable metabolic diseases, namely albinism, alkaptonuria, and 

 phenylketonuria. 



