- The primary goal of my studies was to elucidate the mechanisms for the well-recognized interaction between two nutrients, vitamins E and K. The outcomes from my studies assess mechanisms for adverse effects of vitamin E and provide novel information on mechanisms for vitamin K homeostasis. These findings will provide information relevant for assessing optimal intakes of vitamins E and K.
This dissertation presents studies aimed at evaluating three different mechanisms by which vitamin K status could be decreased by increases in whole body vitamin E concentrations in rats supplemented with vitamin E by subcutaneous injections (100 mg α-tocopherol (α-T)/ kg body weight per day), the model system developed in the Traber lab. The tested mechanisms by which vitamin E leads to reduced vitamin K status were: 1) increasing vitamin K metabolism, 2) decreasing menaquinone-4 (MK-4) synthesis from dietary phylloquinone (PK) and 3) potentiating vitamin K excretion through xenobiotic pathways.
Two approaches were undertaken to evaluate the hypothesis that vitamin E increases vitamin K metabolism. In Aim 1.1, the in vitro omega-hydroxylation of vitamin K by human cytochrome P450 CYP4F2 (expressed in insect microsomes) was tested because CYP4F2 is considered the limiting step in the catabolism of both vitamins. Chapter 2 shows that CYP4F2 more rapidly hydroxylated vitamin K compared with vitamin E. Moreover, vitamin E did not stimulate vitamin K metabolism in vitro. Thus, it is unlikely vitamin E stimulates vitamin K metabolism in vivo by direct interaction with the CYP4F2 enzyme-substrate complex. In Aim 1.2, the in vivo urinary and biliary excretion of vitamin K metabolites was investigated. Chapter 3 shows that α-T-injected rats significantly increased urinary excretion of vitamin E catabolites, but no increases in urinary vitamin K catabolites were found. Chapter 4 shows that α-T-injected rats increased biliary excretion of 5C-aglycone, a major vitamin K catabolite shared by MK-4 and PK. However, the overall in vivo excretion of vitamin K catabolites was not changed when urinary excretion was also taken into account.
Aim 2 evaluated the hypothesis that α-T interferes with the conversion of PK to MK-4 because α-T and PK have similar side-chains. In Aim 2.1, conversion of PK or MN to MK-4 was tested in vivo. Rats were fed semi-purified diets containing equimolar concentrations of either PK or MN for 10 days, then α-T injections were undertaken. Chapter 3 shows that extra-hepatic tissues from α-T injected rats contained significantly lower MK-4 concentrations irrespective of whether the rats were fed PK or MN. These findings show that if vitamin E is interfering with the metabolic mechanism of MK-4 synthesis, then it is not specific to the cleavage of PK's side chain. In Aim 2.2, conversion of deuterium-labeled PK (d₄-PK) to d₄-MK-4 was used to evaluate the extra-hepatic tissue uptake of d₄-PK in α-T-injected rats. Rats were fed semi-purified diets containing equimolar concentrations of d₄-PK similar to my previous study for 10 days then α-T injections were undertaken for 7 days. Chapter 5 shows that total (labeled and unlabeled) vitamin K concentrations decreased in extra-hepatic tissues from α-T injected rats fed d₄-PK. Both d₄-MK-4 and d₄-PK concentrations decreased, suggesting that MK-4 concentrations were dependent upon those of d₄-PK. These findings suggest that PK, and not MN, is the primary substrate for MK-4 synthesis in extra-hepatic tissues. Moreover, both d₄-MK-4 and d₄-PK decreased in α-T-injected rats demonstrating that vitamin E's untoward effect on vitamin K status is likely a mechanism that is shared by both vitamin K forms and not specific to MK-4 synthesis. Recycling of vitamin K from the epoxide was not examined in this study and interference with the recycling mechanism for either PK or MK-4 in α-T injected rats has not been examined.
Vitamin E metabolism is greatly increased in α-T-injected rats by increasing various xenobiotic pathways. Thus, vitamin K status was hypothesized to decrease in α-T-injected rats as a result of the up-regulation of these pathways. As shown in Aim 1, urinary vitamin K metabolite excretion was not increased in α-T-injected rats. In Aim 3.1, the biliary excretion of vitamins E and K were examined to evaluate whether the increased expression in biliary transporters, such as MDR1, led to increased vitamin K and E excretion via the bile. Chapter 4 shows that α-T increased in bile over the week of vitamin E injections and α-CEHC was the major vitamin E form excreted in bile. Although biliary PK secretion was unchanged and biliary MK-4 was undetectable, increased excretion of a major catabolite of both PK and MK-4, 5C-aglycone, was observed. In Aim, 3.2, the gene expression of enzymes and transporters in liver and extra-hepatic tissues as mechanisms involved in regulating their concentrations in these tissues was assessed. In Chapters 3 and 5, increased expression of biliary transporters were observed, one of which is known to bind the vitamin K intermediate MN as its substrate. It is possible other vitamin K catabolites, in addition to 5C-and 7C-aglycone, may have been excreted that were unaccounted for, e.g. MN or vitamin K epoxide metabolites.
In summary, my studies have shown vitamin K status is decreased in α-T-injected rats because PK and MK-4 concentrations are decreased in many extra-hepatic tissues. Although metabolism of vitamin K was not stimulated in response to α-T injections, increased excretion of a vitamin K catabolite was measured in the bile; however it may not account for all of the vitamin K loss observed in tissues. Alternatively, transport of PK and MN to extra-hepatic tissues or MK-4 recycling may have been inhibited in response to vitamin E. Further studies are needed to distinguish between these mechanisms.