Effects of dipyridyl (DP) and salicylaldehyde isonicotinoyl hydrazone (SIH) on the use of endosomal ⁵⁹Fe for heme synthesis and release of ⁵⁹Fe from reticulocytes. One mM (A) or 0.1 mM SIH (B) was included during periods 4 and 5 (Figure 1). At the indicated intervals (during period 5), 2 samples were collected: one was used for measurements of ⁵⁹Fe radioactivity in heme (▪, DP or SIH; □, control), while the other was transferred to 4 mL ice-cold PBS and centrifuged and the ⁵⁹Fe radioactivity in the supernatant measured (•, DP or SIH; ○, control). The values are displayed as the percentage of the total cellular ⁵⁹Fe as it was after the membrane-associated radioiron was displaced by ⁵⁶Fe-Tf (Figure 1, end of period 4).