312 UNITY AND DIVERSITY IN BIOCHEMISTRY 



^-cells which produce insuhn. The latter hormone is a protein (see p. 107) 

 whilst glucagon is a polypeptide in which the amino acid sequence is the 

 following : 



His. ser. glu. gly. thr. phe. thr, ser. asp. tyr. ser. lys. tyr. leu. asp. ser. arg. 

 arg. ala. glu. asp. phe. val. glu. try. leu. met. asp. thr. 



(Brower, Sinn, Staub and Behrens) 



The cells of the thyroid gland fix I ~ ions and oxidize them enzymically to 

 iodine. They contain the specific enzyme systems for the formation of 

 iodotyrosine and iodohistidine, and for the condensation of the iodotyro- 

 sines into iodothyronines, especially the two thyroid hormones, l-3:5:3'- 

 triiodothyronine and L-3:3'-diiodothyronine. Another thyroid hormone 

 and the most important quantitatively is L-thyroxine, derived from the 

 coupling together of two molecules of L-3:5-diiodotyrosine. 



From the mesoderm are formed the cells of the muscles, of connective 

 tissue, the cells of the adrenal cortex, of the gonads and the urinary tract. 



The cell of striated muscle is specialized chiefly for the biosynthesis of 

 myosin, and in the non-myosin fraction of striated muscle fibres we find 

 that the glycolytic enzymes predominate, hence the great importance of 

 aerobic glycolysis in skeletal muscle. The fibres of the myocardium are better 

 provided with mitochondria and the enzyme-systems for respiration and 

 oxidative phosphorylation than those of skeletal muscle. 



The internal framework in mammals consists of connective tissue, 

 cartilage and bone. These structures contain and bound the groupings of 

 specialized epithelial cells — which are localized into organs which is turn 

 are integrated to form the complete organism. From a biochemical point of 

 view, the body framework is a network of connective tissue modified in 

 certain regions to form cartilage and bone. 



The connective tissues are made up of three types of constituents : cells, 

 fibres and ground substance. In ordinary connective tissue, the cells are of 

 two types, histiocytes and fibrocytes. The histiocytes are provided with a 

 number of special biochemical properties notably in the possession of a 

 system able to rupture the a-methene bridge in the haemoglobin of senile 

 (120-130 days old) erythrocytes which they have phagocytized. 



The fibrocytes are differentiated so that they can perform the biosyn- 

 thesis of connective fibres and the ground substance. The collagen fibres, 

 produced by the fibrocytes, differ markedly from the other proteins in the 

 mammalian organism in containing unusually large amounts of glycine and 

 proline, a small amount of histidine and almost undetectable amounts of 

 tryptophane and cystine. Also, collagen contains certain amino acids which 

 are not present in other body proteins (elastin excepted): these are hydroxy- 

 proline and hydroxylysine. Collagen fibres have a special type of physical 

 structure and under the electron microscope a transverse striation can be 



