58 G. Vaes 



calvaria synthesize acid hydrolases during this interval of time; but this synthesis is 

 considerably higher in the resorbing calvaria than in the controls. This is not true for 

 alkaline phenylphosphatase, a nonlysosomal enzyme, which is progressively lost by 

 both control and resorbing calvaria, but more rapidly by resorbing calvaria. In this 

 tissue culture system, PTE thus appears to stimulate both the release (or excretion) 

 and the synthesis of lysosomal acid hydrolases. It stimulates also the excretion of acid 

 into the extracellular spaces. 



Experiments with rats treated with parathyroid extract 



An increased activity of various acid hydrolases (but not of the mitochondrial 

 cytochrome oxidase) (Table 2, A) was observed in the calvaria of newborn rats 



Table 2. Rats treated isjith PTE. A) Enzyme activities in homogenates of calvaria; B) Free 

 activities of enzymes in cytoplasmic extracts of calvaria. — All results expressed as per- 

 centage (mean ± S.D.; N = 6 or 7) of the activities found for paired controls taken as 100"/o. 

 For absolute values in controls, see Vaes and Jacques (1965 a) and Vaes (1965 c) 



Knzyine A B 



^-Glucuronidase 124 ± 17 (p < 0.02) 119 ± 11 (p < 0.01) 



;8-N-Acetvlaminodcoxyglucosidasc Ill ± 16 (N. S.) 135 ± 29 (p < 0.05) 



)5-Galactosidasc I 110± 6 (p = 0.01) i 109 ± 8 (p < 0.05) 



Hyaluronidase (in cytoplasmic extract) 114 ± 13 (p < 0.05) — 



Cathcpsin I 126 ± 10 (p< 0.01) 95 ± 5 (N. S.) 



Acid Dcoxvribonuclease j 138 ± 24 (p < 0.02) 114 ± 7 fp < 0.01) 



Acid Phenylphosphatase I 146 ± 22 (p < 0.01) I 110 ± 16 (N. S.) 



Acid /^-Glycerophosphatase — 108 ± 12 (N. S.) 



Cytochrome Oxidase 99±11(N. S.) — 



treated during 3 days with PTE. This increased concentration of enzymes may be 

 compared with the increased synthesis of acid hydrolases observed in tissue culture 

 under the action of PTE: it suggests that the same phenomenon occurs in vivo and 

 in vitro. Moreover, a larger proportion of the activity found for the acid hydrolases 

 in homogenates of calvaria of rats treated with PTE was found to be in the free state 

 (Table 2, B). This is to be expected if the release of acid hydrolases observed in 

 culture also occurs in vivo, since more soluble enzyme should then be present in the 

 extracellular spaces of the calvaria. 



Conclusions 



The results reported suggest that parathyroid hormone induces a specific release ot 

 lysosomal acid hydrolases by bone cells (osteoclasts?). This could be a critical step of the 

 mechanism whereby the hormone causes bone resorption. Presumably, all lysosomal 

 hydrolases are excreted together in concentrated form at the level of the resorption 

 lacunae, where they exert a concerted eroding action on the matrix before slowly 

 diffusing passively into medium. The same interpretation probably applies to the 

 excretion of hydrogen ions responsible for the acidification of the medium of re- 

 sorbing calvaria: the local pH shift at the resorption sites is probably much greater 

 than that observed in the medium. It could be sufficient to cause a solubilization of 

 bone mineral, leaving the organic matrix uncovered and directly accessible to the 



