A single amino Acid substitution (asparagine to aspartic Acid) between normal (b+) and the common negro variant (a+) of human glucose-6-phosphate dehydrogenase.

Extensive work with human hemoglobins indicates that the typical variant hemoglobin differs from normal hemoglobin by a single amino acid substitution. MAutational studies with E. coli tryptophan synthetase have shown that variants of this protein also owe their origin to single replacement of an amino acid. Using biochemical and genetic recombination techniques, it has been shown that the site of such replacements in the amino acid of the enzyme bears a linear relationship to the site of the mutation within the gene specifying the structure of this enzyme.I Based on this type of evidence, there is general belief among biochemical geneticists that most mutationally altered enzymes are produced by single amino acid substitutions. Since it is difficult to obtain the necessary quantities of pure enzyme proteins for the required biochemical analyses, proof for this hypothesis for mutant enzymes from mammals is not yet available. About 20 variants of human red cell glucose-6-phosphate dehydrogenase (Dglucose-6-phosphate: NADP oxidoreductase, E.C.1.1.1.49) are known. A common variant found in about 18 per cent of American Negro males manifests with rapid electrophoretic mobility and is not associated with enzyme deficiency.2-4 Normal human glucose-6-phosphate dehydrogenase (B+) and the Negro-type variant with normal activity (A+) have beet isolated in homogeneous form, and their molecular weights, subunit molecular sizes, amino acid compositions, enzymatic properties, and serological characteristics have been compared.5' 6 The results indicated that the two enzymes were very similar and that any structural difference between them would be very small. Further studies indicate that the two enzymes differ in a single amino acid-asparagine to aspartic acid. AMaterials and Methods. -Glucose-6-phosphate dehydrogenase: The normal (B+) and the Negro-type variant (A+) enzymes were prepared from blood by methods described previously.5 6 The preparations were homogeneous by physicochemical criteria. Trypsin (EC.3.4.4.4, salt-free, 3 X cryst.), leucine amino peptidase (EC.3.4. 1.1, diisopropylfluorophosphate, DJF1P treated, 80 units/mg), carboxypeptidase A (EC.3.4.2.1, 2X cryst., 40 units/mg), and carboxypeptidase B (EC.3.4.2.2, 100 units/mg) were purchased from Worthingtonl Biochemical Corporation. Leucine amino peptidase was activated by i\4gCl2 before use. Carboxypeptidase A and B were treated with DFP before use.7 Urea was deionized by recrystallization according to the method described by P enesch et al.8 lodoacetic acid was recrystallized from ether and kept at -200. The procedures used for reduction and S-carboxymethylation of the enzyme were essentially those described by Crestfield et al.,9 and the details have been reported. 1 Trypsin digestion: The lyophilized powder of the reduced S-carboxymethylated protein was digested by trypsin at room temperature for four hours. During the