Protoporphyrins

Abstract: The fluorescent porphyrin in the erythrocytes of patients with lead intoxication or with iron deficiency anemia is zinc protoporphyrin that is bound to globin moieties, probably at heme binding sites.

Abstract: Soluble guanylate cyclase [GTP pyrophosphate-lyase (cyclizing), EC 4.6.1.2] purified from bovine lung is markedly activated (30- to 40-fold) by protoporphyrin IX (Ka, 15-25 nM) and is inhibited by hematin (Ki, 3.7 microM) when MgGTP is used as substrate. Guanylate cyclase possesses specific activities (mumol of cGMP per min/mg of protein) of 0.1-0.2 (MgGTP) and 0.3-0.5 (MnGTP) and can attain values of 2-8 (MgGTP) or 1-1.4 (MnGTP) in the presence of protoporphyrin IX. Guanylate cyclase purified in this study contains heme and is activated by nitric oxide and nitrosyl-heme to the same magnitude as that by protoporphyrin IX. With the exception of hematoporphyrin IX, close structural analogs of protoporphyrin IX, including precursors and metabolites, do not activate guanylate cyclase. The insertion of iron into protoporphyrin IX to form heme or hematin renders the metalloporphyrin an inhibitor of unactivated or activated guanylate cyclase. The data suggest that protoporphyrin IX and heme could function to modulate guanylate cyclase activity.

Abstract: Heme, a complex of iron and protoporphyrin IX (PPIX), senses and utilizes oxygen in nearly all living cells. It is an essential component of various hemoproteins, including those involved in oxygen transport and storage (hemoglobin, myoglobin), electron transfer, drug and steroid metabolism (cytochromes), and signal transduction (nitric oxide synthases, guanylate cyclases). The movement of heme into and within cells was thought to occur by diffusion. However, the chemical properties of heme make diffusion too slow to keep pace with biological processes, and accumulation of heme and its pre-cursor porphyrins in membranes can be deleterious. Due to pro-oxidant effects, heme may cause damage to DNA, proteins, the cytoskeleton and membrane lipids. The intracellular localization and concentrations of protoporphyrins and heme are tightly regulated, and elevated levels are linked to pathologic conditions (e.g., anemia, lead poisoning, thalassemias) associated with the formation of membrane lipid-damaging, reactive oxygen species. Until recently a mechanism to transport heme and protoporphyrins into organelles of mammalian cells had not been identified. In this review, we focus on the roles of the recently identified heme/porphyrin transport proteins heme carrier protein 1 (HCP1), FLVCR, Abcg2 and Abcb6 and discuss how these transporters contribute to intracellular heme and porphyrin homeostasis.

Abstract: The hepta-, hexa- and penta-carboxylic porphyrins found in the faeces of rats poisoned with hexachlorobenzene have been separated by high-pressure liquid chromatography and characterized largely by spectroscopie methods. Their structures were confirmed by total synthesis, as part of a programme in which eleven of the fourteen hepta-, hexa- and penta-carboxylic porphyrins derived from uroporphyrin III have now been synthesized as their methyl esters. The four isomeric heptacarboxylic and three of the pentacarboxylic porphyrinogens have been incubated with haemolysates of chicken erythrocytes, and they are all converted into protoporphyrin IX but at different rates. On the basis of this and other evidence we conclude that the decarboxylation of uroporphyrinogen III to coproporphyrinogen III is a stepwise process taking place by a preferred pathway (both in normal and abnormal metabolism); the acetic acid groups are decarboxylated in a sequential clockwise fashion starting with that on the D ring and followed by those on the A, B and C rings. In the poisoned rats the uroporphyrinogen decarboxylase enzyme (or group of enzymes) is probably partially inhibited and the pentacarboxylic porphyrinogen with an acetic acid group on ring C accumulates. The latter is then transformed by a side pathway into dehydroisocoproporphyrinogen and thence into dehydroisocoproporphyrin and its congeners.