ANIMAL COLLAGENASE AND COLLAGEN METABOLISM G79 



incorporation of isotope in the neutral and insoluble fractions. This 

 suggests that either there is an extremely rapid insolubilization of 

 the neutral-extractable collagen, or both are derived independently 

 from a common precursor. 



The fairly constant level of radioactivity obtained by all three 

 collagen fractions over a 20- to 30-hour period is consistent with 

 the observations of Jackson and Bentley ( 1959 ) in guinea pig skin. 

 In their studies only the lower ionic strength cold neutral extracts 

 showed a more rapid "turnover." The meaning is obscure, although 

 the fairly constant high levels of labeled free proline may play a role 

 in our experiments. 



It should be kept in mind that, although induction of meta- 

 morphosis with thyroxin closely resembles the natural process, in- 

 cluding increased thyroid activit\', it is possible that some of the 

 isotope results are a direct effect of thvroxin per se. Though this is 

 unlikely, it has been shown, for example, that thyroid compounds 

 in the range of concentration employed here do stimulate protein 

 synthesis. However, at most this would onlv influence the results in 

 back skin and would not affect the findings associated with tail 

 resorption. 



Collagen Degradation: Detection and Measurement 

 OF A Collagenolytic Enzyme in Tadpole Tissues 



The long-recognized removal of collagen during remodeling has 

 encouraged an intensive quest for an animal collagenase ( see Mandl, 

 1961, for review). To qualify as a true collagenase, the enzyme 

 should digest native collagen at physiologic pH and temperature. 

 The usual approach has been to apply whole tissue homogenates, 

 or fractions therefrom, to a dissolved or solid collagen substrate. 

 Measurements of viscosity decrement or the release of hydroxypro- 

 line or free amino groups have been generally used as assay pro- 

 cedures, analogous to the measurement of bacterial collagenase 

 (Gallop et ah, 1957). The well known proteolytic enzymes such as 

 trypsin, chymotrypsin, the cathepsins, and papain will hardly attack 

 native collagen under physiologic conditions (Mandl, 1961; Gross, 

 1958; Oken and Boucek, 1957; Hodge et al, 1960; Kuhn et al, 1961). 



