Genetical and biochemical studies of the chromogranules in the larval hypodermis of the silkworm, bombyx mori

Abstract

It has been shown by several investigators (Jucci 1932, Hatamura 1943, Shimizu 1943, Doira and Chikushi 1962) that the opaque skin of the silkworm is due to the occurrance of a large amount of uric acid in the hypodermal cells. On the other hand, it was found from our investigation that there is an intimate relation between larval color or oily skin character and the shape, color or amount of chromogranules in the hypodermal cells. Therefore, to confirm whether the chromogranules contain a large amount of uric acid or not experiments were carried out and the following results were obtained. The chromogranules contain soluble protein enclosed in an outer membrane or pellicle, which is mainly composed of proteins and lipids. By the purification procedure described in our previous paper (Tsujita and Sakurai 1963) the chromogranules were isolated. After adding distilled water, the chromogranules were homogenized and centrifuged. The supernatant containing soluble proteins was loaded onto the DEAE (diethylamino-ethyl) cellulose column, and was eluted by phosphate buffer solution at pH 8.0. It could be fractionated into the following three parts. i) 1st fraction contained proteins combining with brown pigment. ii) In the 2nd fraction of the normal strain sample proteins, among them the protein combining with sky-blue fluorescent substance, were eluted, while in the same fraction of the lemon strain sample the protein combining with sepiapterin was collected. The latter protein was eluted before the former as shown in Fig. 2. iii) 3rd fraction contained the protein combining with uric acid and that combining with isoxanthopterin. Furthermore, a fairly large amount of uric acid and isoxanthopterin could be also detected in the insoluble protein which is a major component of the pellicles of chromogranules. Thus, it may be said that the chromogranules play an important role for absorption and maintenance of pteridine compounds as well as uric acid in the larval hypodermis. It was concluded that a number of genes inducing oily skin character of the silkworm larvae are concerned directly or indirectly with the formation of chromogranules. © 1964, The Japanese Society of Sericultural Science. All rights reserved.