It is said that erythrocytes, polymorphonuclear leukocytes and brain cannot produce purines. And the reason given as per this site is:
Human brain tissue has a low level of PRPP glutamyl amidotransferase (reaction Image, Figure 33–2) and hence depends in part on exogenous purines. Erythrocytes and polymorphonuclear leukocytes cannot synthesize 5-phosphoribosylamine (structure III, Figure 33–2) and therefore utilize exogenous purines to form nucleotides.
My question is why do these cells don't produce the required enzymes for the synthesis of purines? What could be the functional significance for such absence?
My attempt : At first I thought that Erythrocytes as such don't have nucleus so they don't need purine synthesis for cell division , but I don't understand why then they need purines from external source( as said in the above site). Also brain cells are neurons and neuroglia, of which former doesn't divide( except in some areas of brain) but they too need purines. The later divide but they don't produce required enzyme. Why?
So my questions are:
Why do cells which don't replicate need purines?( May be for formation of Coenzymes , correct me if Iam wrong)
Why do the cells which divide too don't produce required proteins(enzymes)?