A Gln-281 to Arg substitution in alpha-L-fucosidase is responsible for a common polymorphism detected by isoelectric focusing.

A common polymorphism of human alpha-L-fucosidase consists of three phenotypes (Fu 1, Fu 2, and Fu 2-1) assigned by isoelectric focusing. The phenotypes are determined by two codominant alleles (Fu1 and Fu2). Isozymes with the Fu 2 phenotype have more basic pI than Fu 1, while Fu 2-1 is a mixture of Fu 2 and Fu 1. Recently, a missense mutation (A860-->G) in the alpha-L-fucosidase gene was described that did not affect alpha-L-fucosidase activity. The mutation causes the substitution of Arg (pKaGuan = 12.5) for Gln-281, which has no ionizable side chain. Isoelectric focusing profiles of extracts of COS-1 cells transfected with wild-type or mutant alpha-L-fucosidase cDNAs had phenotypes of Fu 1 and Fu 2, respectively. Next, 20 human lymphoid cell lines were examined for the occurrence of the A860-->G mutation and expression of the Fu 1, Fu 2, and Fu 2-1 phenotypes. Eight lines with Fu 2 were homozygous for the A860-->G mutation; six lines with Fu 1 were homozygous for the normal nucleotide (A860); and six lines with Fu 2-1 were heterozygous. Thus, the A860-->G mutation is the molecular basis for the protein phenotypes and the Fu1 and Fu2 alleles. The normal nucleotide (A860) is responsible for Fu1 and the A860-->G mutation for Fu2.