Radiation-induced breaks of DNA in cultured mammalian cells

Mouse leukemic cells (L5178Y) in suspension culture were irradiated and the extent of single-strand breaks and double-strand cuts of DNA was estimated by sucrose gradient centrifugation. The radiation produced 3.0 single-strand breaks per cell (G(1) stage) per rad and approximately 0.3 double-strand breaks per cell (G(1) stage) per rad.

Generalized gangliosidosis: impaired cleavage of galactose from a mucopolysaccharide and a glycoprotien

We have demonstrated a tenfold (or greater) reduction of the cleavage of galactose from a mucopolysaccharide and a glycoprotein in generalized gangliosidosis, a lysosomal beta-galactosidase deficiency disease. The bony abnormalities in this disease may be due to defective catabolism of mucopolysaccharides in connective tissue.

Effective specific activity dilution in labeled DNA of cultured mammalian cells (L5178Y)

Upon labeling the DNA of cultured mammalian cells with radio-active thymidine it was found that there was a large reduction in the specific activity of radioactive thymidine in DNA when this specific activity was compared to the specific activity of the exogenous supply. This reduction in specific activity may be determined once the effective dilution factor for thymidine in mammalian cells is known. The average effective dilution factor was found to be 2 x 10(-4) M for L5178Y cells in log growth.

Growing points of DNA in cultured mammalian cells

A fraction of DNA, known to be replicating, was examined in the electron microscope. Characteristic branch points were identified as sites at which replication had been taking place. No localized strand separations, nor other structural alterations were discernible at these points.

Generalized gangliosidosis: beta-galactosidase deficiency

A profound deficiency (10- to 30-fold) of beta-galactosidase activity was found in tissues (liver, spleen, kidney, and brain) from two patients with generalized gangliosidosis; this deficiency is demonstrated as a failure to cleave both p-nitrophenyl-beta-D-galactopyranoside and ganglioside GM(1) labeled with C(14) in the terminal galactose. We believe that this enzymic defect is responsible for the accumulation of ganglioside GM(1) and is the fundamental enzyme defect in generalized gangliosidosis.

Replicating units (replicons) of DNA in cultured mammalian cells

Exponentially growing L5178Y mouse leukemic cells were incubated in the presence of 5'-bromodeoxyuridine (BUdR) for about 4 hr, transferred to the nonBUdR-containing medium for a certain period (t hours), and then pulse-labeled with TdR-(3)H for 10 min. When DNA isolated from these cells was subjected to CsCl gradient centrifugation, the (3)H-activity was found to shift gradually from the heavy BUdR-containing peak to the light nonBUdR-containing peak with increasing time t. The average time required for the complete shift of (3)H-activity from the heavy to the light DNA fraction was 2.76 hr. Taking this as the average replicating time and the size of DNA fragments in the present preparation as 1.3 x 10(7) daltons, the rate of replication was found to be 2.1 nucleotides per strand per replicon per sec. By taking the upper limit of the average replicating time as the S period (7.3 hr), various characteristics of the replicating units, such as the lower and upper limits of average size, the average replicating time, the average number of replicating units, etc., were calculated (see Table I).

A comparison of the killing of cultured mammalian cells induced by decay of incorporated tritiated molecules at--196 degrees C

The killing efficiency of tritium disintegrations in frozen mammalian cells labeled with tritiated uridine, histidine, and lysine was compared with the killing efficiency of incorporated tritiated thymidine. In each case, the distribution of tritium in the cells was determined by chemical fractionation as well as by radio-autography. Of all tritium disintegrations, by far the most effective were those occurring in DNA molecules within frozen cells; such incorporated tritium has a killing efficiency of 0.006. When cells were incubated with tritiated uridine for 10 min to label nuclear RNA, the killing efficiency was 0.0015. When the cells were pulse labeled with tritiated uridine and permitted to grow in nonradioactive media for 10 hr before freezing in order to incorporate tritium into cytoplasmic RNA, the killing efficiency was reduced to 0.0010. The results suggest that decay of tritium in nuclear RNA is more effective than that in cytoplasmic RNA. When the cells were labeled with tritiated histidine or lysine for 30 min, tritium atoms were found mainly in the acid soluble rather than in the protein fraction and the killing efficiency in each case was approximately 0.0007. The results of these suicide experiments indicate that the killing efficiency of tritium disintegrations depends on where tritium is located within the cells. Tritium disintegrations in the nucleus are more effective in killing the cell than that in cytoplasm; and tritium disintegrations on DNA in the nucleus is more effective in killing the cell than that of nuclear RNA.

Stationary phase of cultured mammalian cells (L5178Y)

The stationary phase of the mammalian cells L5178Y in culture can be divided into two stages: (a) an early phase characterized by the decline of mitotic index, followed by a stabilization of the cell number, and (b) a late stage, occurring several hours after the flattening of the growth curve, during which dead or dying cells appear in the cultures. The estimates of rates of cell progress showed that the rates from G(1) to S and from G(2) to M were affected in the early stationary phase. The main cause of cessation of increase in cell number in the early stationary phase is resulted from the decline in mitotic index, which is caused by prolongation of the G(2) period. The importance of the G(2) stage in regulating the cell growth is discussed in relation to other known situations in the literature.

Effects of temperature on growth rate of cultured mammalian cells (L5178Y)

L5178Y cells were cultured in vitro at various temperatures. When the cells were in the exponential growth phase, the cells were in the "steady state of growth," i.e., the fraction of cells in the G(1), S, G(2), and M stages and the durations of each stage were constant. The life cycle analysis of the cells in the steady state of growth demonstrated that the G(1) stage and the S stage were affected the most by variation of temperature, and suggested that these two stages have considerable influence on the growth rate of the L5178Y cells. The calculated activation energies were positive in each stage of the life cycle, whereas the entropies of activation were negative throughout. The possible significance of these findings in our search for the regulatory mechanisms of cell growth is discussed.