Some Remarks and Questions on Metabolic Patterns in the Family of Bone Cells 



A most interesting paper on the influence of PTE on bone metabolism and the levels 

 of Nicotinamide nucleotides was given by Van Reen (1965). He determined the 

 pyridine nucleotides in the epiphyseal — metaphyseal area of rabbit femora fluoro- 

 metrically and found quite to his own surprise the NADP content in the tissue of 

 PTE pretreated animals to be increased by a factor 2.25. The NAD content was not 

 changed significantly, NADH and NADPH were decreased by a factor 0.7. 



His surprise does not primarily concern a possible controversy with Hekkelman's 

 data but far more a seeming incompatibility of a PTE induced increase of NADP 

 and the well established PTE induced increase of citrate concentration in bone. (At 

 least this holds as long as one supposes the activity of the NADP dependent Isocitric 

 dehydrogenase to be of importance in the balance of citrate production and oxi- 

 dation.) In fact in Van Reen's opinion the "old" concept of a metabolic block at the 

 Isocitric dehydrogenase level cannot be considered as the final answer in explaining 

 the accumulation of citrate in bone. 



In trying to reflect a little upon this new situation it is good to keep in mind, that 

 in most organs NADP is present mainly in the reduced state, the reverse being true 

 for NAD. In bone tissue, according to van Reen, the ratio NADP/NADPH comes 

 near to unity; under the influence of PTE there is a dramatic shift towards the 

 oxidized state. 



Returning then to Hekkelman's hypothesis concerning the action of PTE, it has 

 been my contribution to show that in Gaillard's system of cultivated mouse radius 

 rudiments, additional NAD or NADP are capable of inhibiting the development of 

 the morphological effect of the hormone. Moreover the same was found to apply for 

 a number of NADP-ase inhibi- 

 tors, suggesting that by arti- 

 ficially keeping up the NADP 

 level the hormone effect is ham- 

 pered in its development (de 

 VooGD van der Straaten, 

 1965). Finally I have been able 

 to demonstrate that PTE in- 

 duces in radius rudiments a 

 decreased binding of i-*C-Nico- 

 tinic Acid, a compound being 

 preeminently the precursor of 

 NAD and NADP; see Fig. 2. 

 Summarizing, my observations 

 seem to be in favour of Hekkel- 

 man's hypothesis and rather 



6 12 18 hours 2i4 



Period of confrontation witti PTE 0.5 I U/ml 



Fig. 2. The effect of PTE 0.5 lU/ml on the Nicotinic acid-Z-'-'C bind- 

 ing capacity of mouse radius rudiments in vitro. The radii were taken 

 from 15-day-old embryo's; the left hand radii were cultivated in 

 standard medium, the corresponding right hand radii being confronted 

 with PTE. In all experiments this period of cultivation was followed 

 ,.^- .... by a 1 hour period of confrontation with labelled Nicotinic acid. The 



dlfhcult to reconcile with Van graph presents the relative activities of the washed bone shafts of 

 Reen's data ^^^ experimental radii, the activities of the corresponding control 



shafts being t.tken as 100 



Against this controversial 

 background there seems to be some need for a "unifying concept". Looking again 

 upon Van Reen's data we meet so to say an "internal discrepancy", possibly even 

 pointing to a way out; see Fig. 3. 



This "discrepancy" is found in the tremendous rise in NADP without any accom- 

 panying rise in the amount of the reduced form. And from the very fact that on the 



