Although pentose phosphate pathway (PPP) operates in all cells, PPP enzyme expression levels are noticeably high in the cells of liver, adipose tissue, adrenal cortex, testis and lactating mammary gland. Erythrocytes also contain high PPP activity. The highest levels of PPP enzymes, particularly glucose 6-phosphate dehydrogenase, are found in neutrophils and macrophages. Explain why the PPP enzymes are highly expressed in these tissues/cells?