Ferric pyridoxal isonicotinoyl hydrazone can provide iron for heme synthesis in reticulocytes

We have examined whether reticulocytes depleted of transferrin might incorporate 59Fe from 59Fe-labelled pyridoxal isonicotinoyl hydrazone (PIH). Transferrin-depleted reticulocytes showed a time-, temperature- and concentration-dependent incorporation of 59Fe when incubated with 20-200 microM 59Fe-PIH. The amount of 59Fe incorporated with 200 microM 59Fe-PIH is equal to or higher than that taken up from transferrin at 20 microM 59Fe concentration. After 60 min about 60% of the 59Fe taken up by the cells is recovered in heme while the remainder is probably still bound to PIH. 1 mM succinyl acetone (a specific inhibitor of heme synthesis) inhibits PIH-mediated incorporation of 59Fe into heme by about 70%, indicating that 59Fe from 59Fe-PIH is incorporated into de novo synthesized protoporphyrin. As is the case with transferrin, erythrocytes do not incorporate 59Fe from 59Fe-PIH. Pretreatment of reticulocytes with pronase does not inhibit their ability to incorporate 59Fe from 59Fe-PIH, suggesting that, unlike the uptake of Fe from transferrin, membrane receptors are not involved in the uptake of Fe-PIH by the cells.