Biochemical studies on amphibian metamorphosis. I. The effect of thyroxine on protein synthesis in the tadpole

Thyroxine has been shown to accelerate the synthesis of carbamyl phosphate synthetase in the liver of Rana catesbeiana. Stimulation of carbamyl phosphate synthetase synthesis by thyroxine appears to be relatively specific because of the following observations: (1) succinoxidase activity decreased during the time that carbamyl phosphate synthetase increased; (2) liver catalase responded more slowly than carbamyl phosphate synthetase to thyroxine; (3) the ratio of biochemical changes/morphological changes was greatly altered during thyroxine-induced metamorphosis. The relationships between the concentration of thyroxine and (1) temperature; (2) duration of exposure of the tadpole to thyroxine; and (3) the activity of carbamyl phosphate synthetase during the induced synthesis of carbamyl phosphate synthetase by thyroxine are discussed. Chloramphenicol and thiouracil partly counteracted the effect of thyroxine on the synthesis of carbamyl phosphate synthetase.

ORGANIZATION AND DISORGANIZATION OF COLLAGEN

The organization of the normal collagen molecule and fibrils is reviewed and the detection, assay, and isolation of a collagenolytic enzyme from amphibian tadpole tissue are described and its possible significance in metamorphosis is discussed

DEVELOPMENTAL STUDIES OF THE DIPTERAN SALIVARY GLAND. II. DNASE ACTIVITY IN CHIRONOMUS THUMMI

The deoxyribonuclease (DNase) activity of the dipteran (Chironomus thummi) salivary gland, measured both enzymatically and immunochemically, increases about 7-fold with the onset of metamorphosis. The increase in DNase activity occurs at a time when the activities of other enzymes and the total protein content are decreasing. The increased DNase activity is followed by glandular destruction. It is suggested that the alterations of this activity may be regulated by the activities of specific chromosomal sites, and that the enzyme may, at least in part, account for the glandular destruction observed at the time of increased enzyme activity.

FINE STRUCTURAL CHANGES IN THE INTESTINAL EPITHELIUM OF THE BULLFROG DURING METAMORPHOSIS

The fine structural changes occurring in the columnar absorbing cells of the intestinal epithelium during metamorphosis of the bullfrog, Rana catesbeiana, have been examined by phase contrast and electron microscopy. Tissue samples taken just posterior to the entrance of the hepatopancreatic duct were fixed in veronal acetate-buffered osmium tetroxide and embedded in methacrylate. Under the action of the metamorphic stimulus (thyroid hormone), specific and characteristic responses were given by differentiated larval cells and undifferentiated basal cells within the same epithelium. The functional larval cells underwent degenerative changes and were retained for a time within the metamorphosing epithelium. Dense bodies appeared and increased in number in association with the loss of normal cell structure. Because of their morphology and time of formation, these bodies have been tentatively identified as lysosomes. Early in metamorphosis the basal cells did not change, but they subsequently proliferated to form a new cell layer beneath the remaining degenerating cells that lined the lumen. After the dying cells were sloughed into the gut, the new epithelium differentiated to form the adult tissue. The columnar epithelial cells of the mature animal differed in their fine structural organization from their larval precursors. Therefore, their adult configuration was molded by the action of the metamorphic stimulus.

CONTROL OF SYNTHESIS OF RNA AND PROTEIN IN DIAPAUSING AND INJURED CECROPIA PUPAE

Injury to diapausing pupae of the cecropia silkworm moth stimulates the synthesis of RNA in all pupal tissues and the synthesis of several blood proteins; such injury also induces the precocious synthesis of a protein, termed "injury protein," which normally appears in the blood during adult development. Actinomycin D, injected in concentrations of 2 micrograms per gram of body weight, blocks the injury-stimulated increase in blood protein synthesis and the injury-induced synthesis of injury protein. However, at concentrations of 0.5 micrograms per gram it prevents the induction of injury-protein synthesis but does not prevent the increased synthesis of other blood proteins. These results suggest that low concentrations of actinomycin may inhibit the synthesis of new kinds of messenger RNA but still permit the continued synthesis of messenger RNA's already in production at the time the actinomycin is injected.

PROTEIN AND NUCLEIC ACID CHANGES DURING GROWTH AND AGING IN THE MOSQUITO

1. Mosquito samples from the entire life span including senescence were analysed for weight, protein, RNA and DNA. 2. The results for each component indicate that maximal values are attained during larval development. No changes were observed during metamorphosis or the adult period. 3. The peak concentration of RNA and DNA was reached on the third day larval age. Protein content and weight were highest on the sixth day. 4. Except for the first 2 days of larval life the DNA/protein ratio was constant. Hence protein content is an index of cell number for this organism. 5. The distribution of the different biochemical components was determined in different body regions and in subcellular fractions. A high concentration of DNA was found in the soluble fraction of larval but not adult homogenates.

FURTHER INVESTIGATIONS OF THE INCORPORATION OF (1-14C)ACETATE INTO THE LIPIDS OF THE SILKWORM BOMBYX MORI L

1. After the injection of sodium [1-(14)C]acetate, the highest incorporation of (14)C into the lipids of the silkworm was observed after 24hr. 2. The specific radioactivity of the palmitic acid fraction was greater and increased more rapidly than that of the stearic acid fraction, which was consistent with the precursor-product relationship to be expected on the basis of current concepts of fatty acid synthesis in vivo. 3. The results indicate the probability of synthesis of lipid components in tissues other than the fat body. 4. Fractionation studies indicate considerable differences in the rate of incorporation of [1-(14)C]acetate into neutral lipids and phospholipids between larvae and pupae as well as among tissues of larvae. 5. The rate of incorporation of [1-(14)C]acetate remains constant throughout pupal development.

TRANSEPIDERMAL POTENTIAL DIFFERENCE: DEVELOPMENT IN ANURAN LARVAE

Skin from bullfrog tadpoles does not show the potential difference characteristic of adult frog skin until several days after the emergence of the forelimbs, presumably because the mechanism for active sodium transport is absent or nonfunctional. Exposure to low concentrations of thyroxine accelerates appearance of the transepidermal potential difference at rates which are related to the dosage. This system may provide a new basis for evaluating the importance of specific biochemical mechanisms in active sodium transport.

TOTAL GLYCOGEN IN THE PEA APHID, ACYRTHOSIPHON PISUM (HARR.) (HOMOPTERA, APHIDIDAE)

In the pea aphid, Acyrthosiphon pisum (Harr.), the amount of glycogen increases steadily as the aphid grows from larval to adult stage. The newly born larva, however, lacks the glycogen reserve.