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Whole body X-ray irradiation was administered to rats during experimentally delayed implantation. The effects of a short (1-day) and a long (9-day) interval between irradiation and the induction of implantation were studied.When one day intervened between irradiation and the induction of implantation, foetal survival decreased with each increase in dose (200-500 r.) and the weights of surviving foetuses were significantly lower than those of the non-irradiated controls.Foetal survival in rats with a 9-day interval between irradiation and the induction of implantation was not significantly different from that of the controls. Foetal weight was lower than that of non-irradiated controls but greater than that of rats with only a 1-day interval.It is concluded that the 9-day interval between irradiation and the induction of implantation allows time for repair of irradiation injury.

The interaction of RNase T1 with calf thymus DNA was studied using uv difference spectroscopy and the effect of the enzyme on DNA melting. There was no indication of RNase T1 binding with native DNA. A prominent difference spectrum for RNase T1 binding with denatured DNA (d-DNA) was observed at pH 5, 25 degrees and low ionic strength (mu = .01 M) which was depressed at higher ionic strength and pH. The normalized difference spectrum at mu = .01 M, pH 5 and 25 degrees can be interpreted as indicating an interaction of an exposed guanine residue directly with the enzyme and a coupling of this process with the "melting" of short folded segments of d-DNA. The apparent association constant calculated per M guanine residues was 2.4 X 10-4 M-1 under these conditions. The results are discussed in reference to comparable studies on the interaction of RNase T1 with RNA and small guanine ligands.

Mh MI 1 I 3 0 C 40 C Tern perature u -LDH-51 J i t LDH-1 4 C LDH-1 h I L @ 25c 30 C Temperature 40 C FIG. 1. Effect of temperature on LDH activity. M = breast muscle, H = heart muscle. I = 3.3 X M pyruvate, h = 6.6 X lo-' M pyruvate. One unit of LDH equals the amount of enzyme utilizing 1 pmole of NADH per minute. FIG. 2. Effect of temperature on LDH 1 and LDH 5 a t high and low pyruvate concentration. The individual isozymes were obtained by elution from starch gel after electrophoresis of heart and muscle supernatants for 16 hr at 4°C. h = 6.6 X lo-' M pyruvate, 1 = 3.3 X M pyruvate. muscle homogenate, and isolated breast muscle LDH 5 both have greater activity at high pyruvate concentration than at low, especially at the physiological body temperature of the chicken (40OC). The LDH in the supernatant of a chicken heart muscle homogenate, and isolated LDH 1 react best at low pyruvate concentration, the difference, however, is less marked at 40OC. These data are in accord with the observation of Cahn et aE ( 3 ) on the differential sensitivity of LDH 1 and LDH 5 to pyruvate concentration.Summary. Spectrophotometric analysis of chick lactate dehydrogenase activity showed that the LDH in breast muscle supernatant and isolated LDH 5 had greater activity at high pyruvate concentration than at low. The difference was more marked at 40°C than at lower temperatures. Heart muscle LDH, and isolated LDH 1 reacted best at low pyruvate concentration. The difference was less marked at 40°C than at lower temperatures.