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Violaxanthin de-epoxidase and zeaxanthin epoxidase catalyze the interconversions between the carotenoids violaxanthin, antheraxanthin and zeaxanthin in plants. These interconversions form the violaxanthin or xanthophyll cycle that protects the photosynthetic system of plants against damage by excess light. These enzymes are the first reported lipocalin proteins identified from plants and are only the second examples of lipocalin proteins with enzymatic activity. This review summarizes the discovery and characterization of these two unique lipocalin enzymes and examines the possibility of other potential plant lipocalin proteins.

9-cis beta-Carotene was extracted from a commercial extract of the algae Dunaliella salina (Betatene), and its actions on proliferation and gene expression were examined in murine 10T1/2 cells and human HaCaT keratinocytes. The 9-cis isomer was less active than all-trans beta-carotene in reducing proliferation and in upregulating expression of connexin 43 in 10T1/2 cells. However, it had comparable ability to suppress carcinogen-induced neoplastic transformation. When tested in HaCaT cells in organotypic culture, it was less active in inducing connexin 43 expression and suppressing expression of keratin K1. In this assay the all-trans isomer was highly active at 10(-6) M, whereas 10(-5) M 9-cis beta-carotene was required to produce a comparable effect. Only small reductions in expression of the basal keratin 5 were seen. All-trans and 9-cis retinoic acids, potential metabolites of beta-carotene isomers, were studied in the same systems. In contrast to the carotenoids, the 9-cis isomer of retinoic acid was approximately 10-fold more active in suppressing neoplastic transformation and inducing connexin 43 expression in both cell types than the all-trans isomer. The retinoic acid isomers were about equipotent in suppressing K1 expression. Cellular levels of 9-cis beta-carotene were approximately 3.5-fold lower than levels of all-trans beta-carotene, suggesting that part, but not all, of this decreased activity of the 9-cis isomer was due to decreased cell uptake. Thus 9-cis beta-carotene is consistently less active than the all-trans isomer; that 9-cis retinoic acid is, in general, much more potent than the all-trans isomer suggests little or no conversion from the carotenoid to the retinoid under these culture conditions.

The human fetal membranes provide a sterile biomechanical container which adjust by growth to mid-pregnancy to the increase in fetal size, and by elasticity to the forceful movements of the fetus. The molecular basis for this elasticity is not known, yet reduced elasticity may lead to their premature rupture and preterm birth, a major problem in perinatal medicine. Classically, elastin confers the property of elastic recoil to elastic fibres which are assembled from a family of tropoelastin precursors. These are covalently cross-linked to form insoluble elastin by formation of desmosine and isodesmosine, catalysed by the enzyme lysyl oxidase. The amnion, chorion and decidua were shown by Northern analysis and RT-PCR to contain detectable levels of tropoelastin mRNA and the mRNA encoding lysyl oxidase. The proteins encoded by these mRNAs were also identified by Western blotting and immunolocalization. Further, insoluble elastin was extracted from the human fetal membranes and shown by comparison to elastin preparations from other elastic tissues to have a reasonable desmosine content. Finally, scanning electron microscopy confirmed the presence of multiple layers of an apparently very thin elastic system in this tissue. This biochemical and histopathologic study has demonstrated therefore that the human fetal membranes synthesize and deposit a novel elastic fibre. The presence of such an elastic system in these tissues provides, for the first time, a probable molecular basis for the elastic properties of this tissue.

Glycoprotein II (GpII) is a heterogenous glycoprotein isolated from the membranes of bovine chromaffin granules in the adrenal medulla. When viewed by two-dimensional electrophoresis this glycoprotein consists of two components, upper (GpIIa) and lower (GpIIb), with a molecular mass of 80,000-100,000 daltons and a pI of 4.2-4.7. NH2-terminal sequence analysis of GpIIa and GpIIb revealed sequence similarity with lysosomal membrane glycoproteins (lamp-1 and lamp-2), which was supported by sequence data of peptides from trypsin and cyanogen bromide digestions. An oligonucleotide probe was used to isolate a cDNA clone encoding the nucleotide sequence of GpIIa. The predicted amino acid sequence of GpIIa shares a 72% identity with the human lamp-1 type protein, which belongs to a highly conserved group of lysosomal-associated membrane glycoproteins (lamp proteins), whose function is still unknown. The COOH-terminal region of GpIIa was identical to the COOH-terminal region of lamp proteins. This COOH-terminal determinant has been demonstrated to be essential for the intracellular targeting of lamp proteins to lysosomes. A synthetic peptide antisera to the COOH-terminal region of GpIIa was used to show that this region is present on purified chromaffin granules and not proteolytically processed. The sequence analysis of GpIIa and immunological data confirm GpII as the secretory granule counterpart of lamp proteins and raise some questions regarding intracellular targeting between lysosomes and secretory granules within the chromaffin cell.