G I Bell

The length of a segment of DNA associated with the human insulin gene, which has been localized to the short arm of chromosome 11, is heterozygous in 63% of 52 individuals analyzed. This polymorphic region is approximately 500 base pairs from the nucleotide encoding the 5' end of insulin mRNA. The polymorphism appears to be due to an insertion or deletion of DNA sequences so that DNA fragments of different length are generated when DNA from a heterozygous individual is digested with selected restriction endonucleases.

There is considerable controversy about the classification and nomenclature of somatostatin receptors. To date, five distinct receptor genes have been cloned and named chronologically according to their respective publication dates, but two were unfortunately given the same appellation (SSTR4). Consensually, a nomenclature for the recombinant receptors has been agreed according to IUPHAR guidelines (sst1, sst2, sst3, sst4, and sst5). However, a more informative classification is to be preferred for the future, employing all classification criteria in an integrated scheme. It is already apparent that the five recombinant receptors fall into two classes or groups, on the basis of not only structure but also pharmacological characteristics. One class (already referred to by some as SRIF1) appears to comprise sst2, sst3 and sst5 receptor subtypes. The other class (SRIF2) appears to comprise the other two recombinant receptor subtypes (sst1 and sst4). This promising approach is discussed but it is acknowledged that much more data from endogenous receptors in whole tissues are needed before further recommendations on somatostatin receptor nomenclature can be made.

Complementary DNA clones encoding a facilitative glucose transporter-like protein have been isolated from human small intestine and muscle cDNA libraries. This 509-amino acid protein has 65.3, 54.3, and 57.5% identity with the previously described human erythrocyte/HepG2, liver, and fetal muscle glucose transporter/transporter-like proteins, respectively. RNA blotting studies indicate that transcripts encoding this protein are very abundant in adult human skeletal muscle and subcutaneous fat. The adult skeletal muscle glucose transporter-like protein was expressed in vitro by cDNA-directed transcription and cell-free translation of the synthetic mRNA. The in vitro-synthesized protein reacted with a monoclonal antibody, 1F8, which recognizes the insulin-regulatable glucose transporter expressed in rat skeletal muscle, heart, and adipocytes. In contrast, in vitro-synthesized erythrocyte/HepG2 and fetal muscle glucose transporters did not react with 1F8. The high levels in adult skeletal muscle and subcutaneous fat of mRNA encoding the adult skeletal muscle glucose transporter and its specific reactivity with monoclonal antibody 1F8 suggest that this protein is the major insulin-regulatable glucose transporter expressed in skeletal muscle and other insulin-responsive tissues.

cDNA clones encoding a glucose transporter-like protein have been isolated from adult human liver and kidney cDNA libraries by cross-hybridization with the human HepG2/erythrocyte glucose transporter cDNA. Analysis of the sequence of this 524-amino acid glucose transporter-like protein indicates that it has 55.5% identity with the HepG2/erythrocyte glucose transporter as well as a similar structural organization. Studies of the tissue distribution of the mRNA coding for this glucose transporter-like protein in adult human tissues indicate that the highest amounts are present in liver with lower amounts in kidney and small intestine. The amounts of glucose transporter-like mRNA in other tissues, including colon, stomach, cerebrum, skeletal muscle, and adipose tissue, were below the level of sensitivity of our assay. The single-copy gene encoding this glucose transporter-like protein has been localized to the q26.1----q26.3 region of chromosome 3.