14 HEMOGLOBIN 



build up hsematin from porphyrin^; (2) that he could, instead of 

 putting in iron, put in cobalt, thus indicating the possibility of pro- 

 ducing a whole series of compounds of porphyrin with different 

 metals; and (3) that by uniting porphyrin with copper he produced a 

 substance which seemed to be, but was not quite identical with the 

 pigment turacin which is extracted from the feathers of the South 

 African bird Turaco. This pigment has always evoked a mild interest 

 because, being somewhat soluble in dilute alkali, it disappears from 

 the feathers during the rainy season. The nest is contaminated with 

 the alkaline excrement of the bird; and the excrement becoming 

 moistened by the rain dissolves the turacin, 



Laidlaw, then, extended the field of interest in porphyrin, on the 

 chemical side, by showing that the iron-porphyrin compound was only 

 one of a number of " metaUoporphyrins " and, on the biological side, 

 by showing that the copper porphyrin was responsible for a pigment 

 of very different biological significance from haemoglobin. 



In both fields, that of the metaUoporphyrins and that of turacin, 

 great advances have been made within recent years. 



With regard to metaUoporphyrins Schulz(9) in 1904 (the same 

 year in which Laidlaw prepared the copper and cobalt porphyrins) 

 made the zinc body, Milroy(iO) in 1909 manufactured a nickel and 

 a tin metalloporphyrin, and now the compoimds with aluminium 

 silver, sodium and potassium have all been carefully studied by Robert 

 HiU{ii). Of these only three can be oxidised and reduced, namely the 

 compounds of cobalt (as shown by Laidlaw), iron and manganese. 



To pass to turacin, which was first described by Church (12). A few 

 lines back I said that the turacin of the feathers was dissolved by 

 dUute alkali: that statement needs some justification. It is true 

 that when the feathers- are treated with alkali pigment leaves the 

 feathers and appears in the alkali. But is the process merely one of 

 solution ? or is the turacin changed into something else ? The reason 

 for suspicion is that the spectrum of the pigment observed in the alkali 

 differs from that seen in the feathers. What then are these spectra, 

 and how are they related ? . These matters have been studied by 

 KeUin (i3) and are of great importance on account of the light which 

 they throw upon the constitution of pigments of much wider dis- 

 tribution than turacin. 



^ The difference between the different porphyrins was not recognised at the time 

 of Laidlaw's work, actually it was ooporphyrin that he obtained from reduced hsemo- 

 globin. 



