An Erythroid-Specific Exon Is Present in the Human Hexokinase Gene

To the Editor:

Human hexokinase type I (EC 2.7.1.1) is the predominant glucose-phosphorylating enzyme in red blood cells. By a number of methods1 it has been proved, and is now widely accepted, that this enzyme is largely heterogeneous and present in multiple molecular forms. Hexokinase subtypes have similar kinetic properties but a different age-dependent decay and a different intracellular distribution in reticulocytes.

It is presently unknown if the multiple hexokinase subtypes reflect posttranslational modifications or different gene products. We previously showed that, at least in human placenta, the heterogeneity of hexokinase type I is caused by the presence of truncate forms arising postsynthetically.2 

In the February issue of BLOOD, Murakami and Piomelli3reported evidence for a red cell–specific hexokinase cDNA containing a unique sequence of 60 nucleotides at the beginning of the coding region. We have now identified this red cell–specific hexokinase sequence in the human genome and found that it is located 3.1 kb upstream from the somatic exon 1 (GenBank accession number AF016350). Determination of the splice-junction by direct sequencing confirmed the hypothesis that a true hexokinase isozyme may exist in humans, likely as a product of an alternative splicing event. However, human erythrocytes show a multiplicity of forms that cannot be explained only on the bases of two alternative hexokinase isoforms.4,5Thus, the origin of hexokinase multiplicity remains at least in part to be determined.

Finally, we would like to note that expression of recombinant human hexokinase type I lacking the porin-binding domain results in an enzyme with normal kinetic and regulatory properties (Bianchi et al, submitted for publication). Thus, in cases of hexokinase mutations with altered enzymatic properties, the mutation must be searched for downstream of exon 1. This exon could instead confer stability to the enzyme by favoring binding to intracellular organelles and be responsible for enzyme defects with accelerated in vivo hexokinase decay.6